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ASTRONOMY 



FOR THE USE OF 



SCHOOLS A N D ACADEMIES. 



T. A. GIL LET. 



FFNFESSOF IF FHYOIOS IN THE NORMAL 0:LLEOE C? THE OILY 7F NEW VCRK. 



W. J. -10LFE. 

FORMERLY HEAD MASTER OF THE HIGH SCHOOL, 

CAMBRIDGE, MASS. 




POTTER. AINSWORTH, & CO., 

NEW YORK AND CHICAGO. 
1882. 



tfi 






Copyright by 
J. A. GILLET and W. J. ROLFE, 

1882. 



jFranfcitn press : 

RAND. AVERY, AND COMPANY, 
BOSTON. 



PREFACE. 



It has been the aim of the authors to give in 
this little book a brief, simple, and accurate account 
of the heavens as they are known to astronomers 
of the present day. It is believed that there is 
nothing in the book beyond the comprehension of 
the pupil of the ordinary high school or academy, 
and that it contains all the information on the 
subject of astronomy that is needful to a person 
of ordinary culture. The authors have carefully 
avoided dry and abstruse mathematical calculations, 
yet they have sought to make clear the methods by 
which astronomers have gained their knowledge of 
the heavens. The various kinds of telescopes and 
spectroscopes have been described, and their use in 
the study of the heavens has been fully explained. 

The cuts with which the book is illustrated have 
been drawn from all available sources ; and it is be- 
lieved that they excel in number, freshness, beauty, 
and accuracy those to be found in any similar work. 
The lithographic plates are, with a single exception, 
reductions of the plates prepared at the Observa- 
tory at Cambridge, Mass. The remaining litho- 
graphic plate is a reduced copy of Professor Lang- 
ley's celebrated sun-spot engraving. Many of the 
views of the moon are from drawings made from 



iv PREFACE. 

the photographs in Carpenter and Nasmyth's work 
on the moon. The majority of the cuts illustrating 
the solar system are copied from the French edition 
of Guillemin's " Heavens." Most of the remainder 
are from Lockyer's " Solar Physics," Young's "Sun," 
and other recent authorities. The cuts illustrating 
comets, meteors, and nebulae, are nearly all taken 
from the French editions of Guillemin's " Comets " 
and Guillemin's " Heavens." 

The first division of the work contains an account 
of the real and apparent motions of the heavenly 
bodies. The method of presentation in this sec- 
tion is that which one of the authors has found, 
during a long experience, to be most successful 
in the class-room. 

The second division contains an account of the 
solar system. In this section the authors have been 
indebted for portions of their material to Guillemin's 
" Heavens " and " Comets," Newcomb's " Popular 
Astronomy," Lockyer's " Solar Physics," and Young's 
"Sun." 

The third division contains a description of the 
stellar universe, including an account of the aggre- 
gation of the stars into constellations and clusters, 
of the variability, motion, and distance of the stars, 
of nebulae, and of the structure of the stellar uni- 
verse. The material of this section has been derived 
from a great variety of sources, including some of 
those already mentioned. 

This book is an entirely new and independent 
work, and in no sense a revision of the " Astron- 
omy " of the "Cambridge Course of Physics," by 
the same authors. For the convenience of teach- 



PREFACE. V 

ers whose time is too limited to allow of their 
taking the whole book, the portions which, in the 
opinion of the authors, can best be spared in a brief 
course, are printed in finer type. It is hardly neces- 
sary to say that the abridgment may be made in 
other ways, according to the judgment or taste of the 
teacher. 

It may be added that all the works to which the 
authors have acknowledged their indebtedness will 
be helpful to the teacher in connection with this 
book. 



CONTENTS. 



PAGE 

I. THE CELESTIAL SPHERE .... 3 

II. THE SOLAR SYSTEM 41 

I. THEORY OF THE SOLAR SYSTEM . . 41 

The Ptolemaic System 41 

The Copernican System 44 

Tycho Brahe's System 44 

Kepler's System 44 

The Newtonian System ; 48 

II. THE SUN AND PLANETS 53 

I. The Earth 53 

Form and Size 53 

Day and Night 57 

The Seasons 64 

Tides 68 

The Day and Time 74 

The Year 78 

Weight of the Earth and Precession . S^ 

II. The Moon 86 

Distance, Size, and Motions .... 86 
The Atmosphere of the Moon . . . .109 

The Surface of the Moon . . . . 114 

III. Inferior and Superior Planets . . . . 130 

Inferior Planets . . . . . 130 

Superior Planets 134 

vii 



Vill CONTENTS. 

PAGE 

IV. The Sun 140 

I. Magnitude and Distance of the Sun . . 140 
II. Physical and Chemical Condition of the 

Sun 149 

Physical Condition of the Sun . . .149 

The Spectroscope 152 

Spectra 158 

Chemical Constitution of the Sun . 164 

Motion at the Surface of the Sun . . 168 

III. The Photosphere and Sun-Spots . . . 175 

The Photosphere 175 

Sun-Spots 179 

IV. The Chromosphere and Prominences . . 196 
V. The Corona 204 

V. Eclipses 210 

VI. The Three Groups of Planets . . . . 221 

I. General Characteristics of the Groups . 221 

II. The Inner Group of Planets . . . 225 

Mercury 225 

Venus 230 

Mars 235 

III. The Asteroids 241 

IV. Outer Group of Planets 244 

Jupiter 244 

The Satellites of Jupiter .... 250 

Saturn 255 

The Planet and his Moons . . . 255 

The Rings of Saturn . . . . 261 

Uranus 269 

Neptune '271 

VII. Comets and Meteors 274 

I. Comets 274 

General Phenomena of Comets . . - . 274 

Motion and Origin of Comets ... 281 

Remarkable Comets 290 



CONTENTS. IX 

I. Comets, continued. page 

Connection between Meteors and Comets, 300 
Physical and Chemical Constitution of 

Comets 314 

II. The Zodiacal Light 318 

III. THE STELLAR UNIVERSE .... 322 

I. General Aspect of the Heavens . . . 322 

II. The Stars 330 

The Constellations 330 

Clusters . 350 

Double and Multiple Stars . . . . 355 

New and Variable Stars 358 

Distance of the Stars ....... 364 

Proper Motion of the Stars .... 365 

Chemical and Physical Constitution of the 

Stars 371 

III. Nebula i>73 

Classification of Nebula .... 373 

Irregular Nebulae 376 

Spiral Nebulae 384 

The Nebular Hypothesis 391 

IV. The Structure of the Stellar Universe . 396 



ASTRONOMY. 



ASTRONOMY. 



i. 

THE CELESTIAL SPHERE. 

i . The Sphere. — A sphere is a solid figure bounded by 
a surface which curves equally in all directions at every 
point. The rate at which the surface curves is called the 
curvature of the sphere. The smaller the sphere, the greater 
is its curvature. Every point on the surface of a sphere is 
equally distant from a point within, called the centre of 
the sphere. The circumference of a sphere is the distance 
around its centre. The diameter of a sphere is the dis- 
tance through its centre. The radius of a sphere is the 
distance from the surface to the centre. The surfaces of 
two spheres are to each other as the squares of their radii 
or diameters.; and the volumes of two spheres are to each 
other as the cubes of their radii or diameters. 

Distances on the surface of a sphere are usually denoted in 
degrees. A degree is -^\-§ of the circumference of the sphere. 
The larger a sphere, the longer are the degrees on it. 

A curve described about any point on the surface of a 
sphere, with a radius of uniform length, will be a circle. 
As the radius of a circle described on a sphere is a curved 
line, its length is usually denoted in degrees. The circle 
described on the surface of a sphere increases with the 
length of the radius, until the radius becomes 90 , in which 
case the circle is the largest that can possibly be described 

3 



ASTRONOMY, 



_?A 



on the sphere. The largest circles that can be described on 
the surface of a sphere are called great circles, and all other 
circles small circles. 

Any number of great circles may be described on the sur- 
face of a sphere, since any point on the sphere may be used 
for the centre of the circle. The plane of every great circle 
passes through the centre of the sphere, while the planes of 
all the small circles pass through the sphere away from the 
centre. All great circles on the same sphere are of the same 
size, while the small circles differ in size according to the dis- 
tance of their planes from the centre of the sphere. The far- 
ther the plane of a circle is from the centre of the sphere, the 
smaller is the circle. 

By a section of a sphere we usually mean the figure of the 
surface formed by the cutting : by a plane section we mean one 

whose surface is plane. Even- 
plane section of a sphere is 
a circle. When the section 
passes through the centre of 
the sphere, it is a great 
circle: in every other case 
the section is a small circle. 
Thus. AN and SB (Fig. i) 
are small circles, and MM 
and SNare large circles. 

In a diagram representin^- 
a sphere in section, all the 
circles whose planes cut the 
section are represented by 
straight lines. Thus, in Fig. 2. we have a diagram representing 
in section the sphere of Fig. 1. The straight lines AN, SB. 
MM', and S N, represent the corresponding circles of Fig. 1. 

The axis of a sphere is the diameter on which it rotates. 
The poles of a sphere are the ends of its axis. Thus, sup- 
posing the spheres of Figs. 1 and 2 to rotate on the diame- 
ter PP, this line would be called the axis of the sphere, 
and the points />and P' the poles of the sphere. A great 




ASTRONOMY. 



5 




circle. MM\ situated half way between the poles of a 
sphere, is called the equator of the sphere. 

Every great circle of a sphere has two poles. These are 
the two points on the sur- 
face of the sphere which lie 
90 away from the circle. 
The poles of a sphere are 
the poles of its equator. 

2. The Celestial Sphere. 
— The heavens appear to 
have the form of a sphere, 
whose centre is at the eye 
of the observer ; and all the 
stars seem to lie on the sur- 
face of this sphere. This 
form of the heavens is a 
mere matter of perspective. The stars are really at very 
unequal distances from us : but they are all seen project- 
ed upon the celestial 
sphere in the direc- 
tion in which they 
happen to lie. Thus, 
suppose an observer 
situated at C (Fig. 3). 
stars situated at a, b, 
it e, f\ and g, would 
be projected upon the 
sphere at A, B. D, E, 
F. and G, and would 
appear to lie on the 
surface of the heav- 
ens. 

3 . The Ho rizo n . — 
Only half of the celestial sphere is visible at a time. The 
plane that separates the visible from the invisible portion is 




ASTRONOMY. 




called the horizon. This plane is tangent to the earth at 
the point of observation, and extends indefinitely into space 
in every direction. In Fig. 4, E represents the earth, O the 

point of observation, and SN 
the horizon. The points on 
the celestial sphere directly 
above and below the observer 
are the poles of the horizon. 
J * T They are called respectively 
the zenith and the nadir. No 
two observers in different 
parts of the earth have the 
same horizon ; and as a per- 
son moves over the earth he 
carries his horizon with him. 
The dome of the heavens appears to rest on the earth, 
as shown in Fig. 5. This is because distant objects on 
the earth appear pro- 
jected against the 
heavens in the di- 
rection of the hori- 
zon. 

The sensible hori- 
zon is a plane tan- 
gent to the earth 
at the point of ob- 
servation. The ra- 
tional horizon is a 
plane parallel with 
the sensible horizon, 
and passing through 
the centre of the 




Fig. 



earth. As it cuts the celestial sphere through the cen- 
tre, it forms a great circle. SN (Fig. 6) represents 
the sensible horizon, and S' N' the rational horizon. 



ASTRONOMY. 




Although these two horizons are really four thousand miles 
apart, they appear to meet at the distance of the celestial 
sphere ; a line four thousand miles long at the distance of 
the celestial sphere becom- 
ing a mere point, far too 
small to be detected with 
the most powerful tele- 
scope. 

4. Rotation of the Celes- 
tial Sphere. — It is well 
known that the sun and the 
majority of the stars rise in 
the east, and set in the west. 
In our latitude there are Fig. 6. 

certain stars in the north which never disappear below the 
horizon. These stars are called the circumpolar stars. A 
close watch, however, reveals the fact that these all appear 
to revolve around one of their number called the pole star, 

in the direction indi- 
cated by the arrows in 
Fig. 7. In a word, the 
whole heavens appear 
to rotate once a day. 
from east to west, 
about an axis, which 
is the prolongation of 
the axis of the earth. 
The ends of this axis 
are called the poles 
^Mj^^^^ ^^di^m °f tne heavens : and 

Fig. 7. the great circle of the 

heavens, midway between these poles, is called the celestial 
equator, or the equinoctial This rotation of the heavens 
is apparent only, being due to the rotation of the earth 
from west to east. 




8 



ASTRONOMY. 



5. Diurnal Circles. — In this rotation of the heavens, the 
stars appear to describe circles which are perpendicular to 
the celestial axis, and parallel with the celestial equator. 
These circles are called diurnal circles. The position of 

the poles in the heavens 
and the direction of the 
diurnal circles with reference 
to the horizon, change with 
the position of the observer 
on the earth. This is owing 
to the fact that the horizon 
changes with the position of 
the observer. 

When the observer is north 
of the equator, the north 
pole of the heavens is ele- 




Fig. 8. 



vated above the horizon, and the south pole is depressed 
below it, and the diurnal circles are oblique to the horizon, 
leaning to the south. This case is represented in Fig. 8, in 
which P P' represents the 
celestial axis, E Q the celes- 
tial equator, SiVthe horizon, 
and ab, c JV, de, fg, Sh, 
k /, diurnal circles. O is the 
point of observation, Z the 
zenith, and Z' the nadir. 

When the observer is south 
of the equator, as at O in 
Fig. 9, the south pole is 
elevated, the north pole de- 
pressed, and the diurnal cir- 
cles are oblique to the horizon, leaning to the north. When 
the diurnal circles are oblique to the horizon, as in Figs. 8 
and 9, the celestial sphere is called an oblique sphere. 

When the observer is at the equator, as in Fig. 10, the 




ASTRONOMY. 




poles of the heavens are on the horizon, and the diurnal 
circles are perpendicular to the horizon. 

When the observer is at one of the poles, as in Fig. n. 
the poles of the heavens 
are in the zenith and the 
nadir, and the diurnal cir- 
cles are parallel with the 
horizon. 

6. Elevation of the Pole 
and of the Eguinoctial. — 
At the equator the poles 
of the heavens lie on the 
horizon, and the celestial 
equator passes through the 
zenith. As a person moves 
north from the equator, his 
zenith moves north from the celestial equator, and his hori- 
zon moves down from the north pole, and up from the south 
pole. The distance of the zenith from the equinoctial, and 

of the horizon from the celes- 
tial poles, will always be equal- 
to the distance of the observ- 
er from the equator. In other 
words, the elevation of the 
pole is equal to the latitude 
of the place. In Fig. 12, O 
is the point of observation. 
Z the zenith, and SN the 
horizon. N P. the elevation 
of the pole, is equal to ZE. 
the distance of the zenith 
from the equinoctial, and to 




Fig. 11. 



the distance of O from the equator, or the latitude of the 
place. 

Two angles, or two arcs, which together equal 90 , are 



IO 



ASTRONOMY. 



said to be complements of each other. ZE and E S in 
Fig. 12 are together equal to 90° : hence they are comple- 



ments of each other. ZE 




7. Four Sets of Stars. - 
there are four sets of stars 

Fig- 13- 

(1) The stars in 
the neighborhood of 
the elevated pole 
never set It will 
be seen from Fig. 
13. that if the dis- 
tance of a star from 
the elevated pole 
does not exceed the 
elevation of the pole, 
or the latitude of 
the place, its diurnal 
circle will be wholly 
above the horizon. 



is equal to the latitude of the 
place, and E S is the eleva- 
tion of the equinoctial above 
the horizon : hence the ele- 
vation of the equinoctial is 
equal to the complement of 

7f the latitude of the place. 

Were the observer south 
of the equator, the zenith 
would be south of the equi- 
noctial, and the south pole 
of the heavens would be the 
elevated pole. 

At most points of observation 
These four sets are shown in 




Fig. 13. 



As the observer approaches the equator, the elevation of 
the pole becomes less and less, and the belt of circumpolar 
stars becomes narrower and narrower : at the equator it 



ASTRONOMY. 1 1 

disappears entirely. As the observer approaches the pole, 
the elevation of the pole increases, and the belt of circum- 
polar stars becomes broader and broader, until at the pole 
it includes half of the heavens. At the poles, no stars rise 
or set, and only half of the stars are ever seen at all. 

(2) The stars in the neighborhood of the depressed pole 
never rise. The breadth of this belt also increases as the 
observer approaches the pole, and decreases as he approaches 
the equator, to vanish entirely when he reaches the equator. 
The distance from the depressed pole to the margin of this 
belt is always equal to the latitude of the place. 

(3) The stars in the neighborhood of the equinoctial, on 
the side of the elevated pole, set, but are above the horizon 
longer than they are below it. This belt of stars extends 
from the equinoctial to a point whose distance from the 
elevated pole is equal to the latitude of the place : in other 
words, the breadth of this third belt of stars is equal to 
the complement of the latitude of the place. Hence this 
belt of stars becomes broader and broader as the observer 
approaches the equator, and narrower and narrower as he 
approaches the pole. However, as the observer approaches 
the equator, the horizon comes nearer and nearer the celes- 
tial axis, and the time a star is below the horizon becomes 
more nearly equal to the time it is above it. As the observ- 
er approaches the pole, the horizon moves farther and far- 
ther from the axis, and the time any star of this belt is 
below the horizon becomes more and more unequal to the 
time it is above it. The farther any star of this belt is from 
the equinoctial, the longer the time it is above the horizon, 
and the shorter the time it is below it. 

(4) The stars which "are in the neighborhood of the 
equinoctial, on the side of the depressed pole, rise, but are 
below the horizon longer than they are above it. The width 
of this belt is also equal to the complement of the latitude 
of the place. The farther any star of this belt is from the 



12 



ASTRONOMY. 




equinoctial, the longer time it is below the horizon, and the 
shorter time it is above it ; and, the farther the place from 
the equator, the longer every star of this belt is below the 
horizon, and the shorter the time it is above it. 

At the equator every star 
is above the horizon just 
half of the time ; and any 
star on the equinoctial is 
above the horizon just half 
of the time in every part of 
the earth, since the equinoc- 
tial and horizon, being great 
circles, bisect each other. 

8. Vertical Circles. — 
Great circles perpendicular 
to the horizon are called ver- 
tical circles. All vertical circles pass through the zenith and 
nadir. A number of these circles are shown in Fig. 14, 
in which 6 1 E N W represents the horizon, and Z the zenith. 

The vertical circle which 
passes through the north and 
south points of the horizon 
is called the meridian ; and 
the one which passes through 
the east and west points, the - 
prime vertical. These two 
circles are shown in Fig. 15 ; 
SZN being the meridian, 
and E Z JV the prime verti- 
cal. These two circles are 
at right angles to each other, 
or 90 apart ; and consequently they divide the horizon into 
four quadrants. 

9. Altitude and Zenith Distance. — The altitude of a 
heavenly body is its distance above the horizon, and its 




ASTRONOMY. 1 3 

zenith distance is its distance from the zenith. Both the 
altitude and the zenith distance of a body are measured on 
the vertical circle which passes through the body. The alti- 
tude and zenith distance of a heavenly body are comple- 
ments of each other. 

10. Azimuth and Amplitude. — Azimuth is distance meas- 
ured east or west from the meridian. When a heavenly 
body lies north of the prime vertical, its azimuth is meas- 
ured from the meridian on the north ; and. when it lies 
south of the prime vertical, its azimuth is measured from the 
meridian on the south. The azimuth of a body can, there- 
fore, never exceed 90 . The azimuth of a body is the angle 
which the plane of the vertical circle passing through it 
makes with that of the meridian. 

The amplitude of a body is its distance measured north 
or south from the prime vertical. The amplitude and azi- 
muth of a body are complements of each other. 

11. Alt-azimuth Instrument. — An instrument for meas- 
uring the altitude and azimuth of a heavenly bod)' is called 
an alt-azimuth instrument. One form of this instrument is 
shown in Fig. 16. It consists essentially of a telescope 
mounted on a vertical circle, and capable of turning on a 
horizontal axis, which, in turn, is mounted on the vertical 
axis of a horizontal circle. Roth the horizontal and the 
vertical circles are graduated, and the horizontal circle is 
placed exactly parallel with the plane of the horizon. 

When the instrument is properly adjusted, the axis of the 
telescope will describe a vertical circle when the telescope 
is turned on the horizontal axis, no matter to what part of 
the heavens it has been pointed. 

The horizontal and vertical axes carry each a pointer. 
These pointers move over the graduated circles, and mark 
how far each axis turns. 

To find the azimuth of a star, the instrument is turned 
on its vertical axis till its vertical circle is brought into the 



14 



ASTRONOMY. 



plane of the meridian, and the reading of the horizontal 
circle noted. The telescope is then directed to the star by 
turning it on both its vertical and horizontal axes. The 







Fig. 1 6. 

reading of the horizontal circle is again noted. The differ- 
ence between these two readings of the horizontal circle 
will be the azimuth of the star. 



ASTRONOMY. 



IS 



To find the altitude of a star, the reading of the vertical 
circle is first ascertained when the telescope is pointed hori- 
zontally, and again when the telescope is pointed at the star, 
The difference between these two readings of the vertical 
circle will be the altitude of the star. 

12. The Vernier. — To enable the observer to read the 
fractions of the divisions on the circles, a device called a 
vernier is often employed. It consists of a short, graduated 
arc, attached to the end of an arm c (Fig. 17), which is 
carried by the axis, and turns with the telescope. This arc 
is of the length of nine divisions on the circle, and it is 
divided into ten equal parts. If o of the vernier coincides 
with any division, say 6. of the circle. 1 of the vernier will 
be t 1 q of a division to 
the left of 7. 2 will be 
•^ of a division to the 



left of 8. 3 will be 



3 
10 




Fig. 17. 



of a division to the left 
oi 9. etc. Hence, when 
1 coincides with 7. o 
will be at 6-^ : when 2 
coincides with 8. o will 
be at 6 T - - : when 3 coincides with 9. o will be at 6^. etc. 

To ascertain the reading of the circle by means of the 
vernier, we first notice the zero line. If it exactly coin- 
cides with any division of the circle, the number of that 
division will be the reading of the circle. If there is not 
an exact coincidence of the zero line with any division of 
the circle, we run the eye along the vernier, and note which 
ot its divisions does coincide with a division of the circle. 
The reading of the circle will then be the number of the 
first division on the circle behind the o of the vernier, and 
a number of tenths equal to the number of the division of 
the vernier, which coincides with a division of the circle. 
For instance, suppose o of the vernier beyond 6 of the 



i6 



ASTRONOMY. 



circle, and 7 of the vernier to coincide with 13 of the 
circle. The reading of the circle will then be 6 T V 

13. Hour Circles. — Great circles perpendicular to the 
celestial equator are called hour circles. 



These circles all 




pass through the poles of 
the heavens, as shown in 
Fig. 18. E Q is the celes- 
tial equator, and P and 
P r are the poles of the 
heavens. 

The point A on the 
equinoctial (Fig. 19) is 
called the vernal equinox, 
or the first point of Aries. 
The hour circle, A P P\ 
which passes through it, is 

called the equinoctial colure. 

14. Declination and Right Ascension. — The declination 

of a heavenly body is its distance north or south of the 

celestial equator. The polar 

distance of a heavenly body 

is its distance from the nearer 

pole. Declination and polar 

distance are measured on 

hour circles, and for the same 

heavenly body they are com- 
plements of each other. 
The right ascension of a 

heavenly body is its distance 

eastward from the first point 

of Aries, measured from the 

equinoctial colure. It is equal to the arc of the celestial 

equator included between the first point of Aries and the 

hour circle which passes through the heavenly body. As 

right ascension is measured eastward entirely around the 




ASTRONOMY, 



17 



celestial sphere, it may have any value from o° up to 360 . 
Right ascension corresponds to longitude on the earth, and 
declination to latitude. 

15. The Meridian Circle. — The right ascension and 
declination of a heavenly body are ascertained by means of 
an instrument called the meridian circle, or transit instru- 
ment. A side-view of this instrument is shown in Fig. 20. 




It consists essentially of a telescope mounted between two 
piers, so as to turn in the plane of the meridian, and carry- 
ing a graduated circle. The readings of this circle are 
ascertained by means of fixed microscopes, under which it 
turns. A heavenly body can be observed with this instru- 
ment, only when it is crossing the meridian. For this reason 
it is often called the transit circle. 

To find the declination of a star with this instrument, we 



1 8 ASTRONOMY. 

first ascertain the reading of the circle when the telescope 
is pointed to the pole, and then the reading of the circle 
when pointed to the star on its passage across the meridian. 
The difference between these two readings will be the polar 
distance of the star, and the complement of them the decli- 
nation of the star. 

To ascertain the reading of the circle when the telescope 
is pointed to the pole, we must select one of the circum- 
polar stars near the pole, and then point the telescope to 
it when it crosses the meridian, both above and below the 
pole, and note the reading of the circle in each case. The 
mean of these two readings will be the reading of the circle 
when the telescope is pointed to the pole. 

1 6. Astronomical Clock. — An astronomical clock, ox si- 
dereal clock as it is often called, is a clock arranged so as 
to mark hours from i to 24, instead of from 1 to 12, as in 
the case of an ordinary clock, and so adjusted as to mark 
o when the vernal equinox, or first point of Aries, is on the 
meridian. 

As the first point of Aries makes a complete circuit of 
the heavens in twenty-four hours, it must move at the rate 
of 1 5 an hour, or of i° in four minutes : hence, when the 
astronomical clock marks t, the first point of Aries must be 
15 west of the meridian, and when it marks 2, 30 west of 
the meridian, etc. That is to say, by observing an accurate 
astronomical clock, one can always tell how far the meridian 
at any time is from the first point of Aries. 

17. How to find Right Ascension with the Meridian 
Circle. — To find the right ascension of a heavenly body, 
we have merely to ascertain the exact time, by the astro- 
nomical clock, at which the body crosses the meridian. If 
a star crosses the meridian at 1 hour 20 minutes by the 
astronomical clock, its right ascension must be 19 ; if at 
20 hours, its right ascension must be 300 . 

To enable the observer to ascertain with great exactness 



ASTRONOMY. 



*9 



the time at which a star crosses the 
equidistant and parallel spider- 
lines are stretched across the 
focus of the telescope, as shown in 
Fig. 2 1 . The observer notes the 
time when the star crosses each 
spider-line ; and the mean of all 
of these times will be the time 
when the star crosses the meridi- 
an. The mean of several obser- 
vations is likely to be more nearly 
exact than any single observation. 
18. The Equatorial Telescope. 



meridian, a number of 





Fig. 22. 

fixed in any declination, and then t 



The equato?ial tele- 
scope is mounted on 
two axes, — one par- 
allel with the axis of 
the earth, and the 
other at right angles 
to this, and therefore 
parallel with the plane 
of the earth's equator. 
The former is called 
the polar axis, and 
the latter the declina- 
tion axis. Each axis 
carries a graduated 
circle. These circles 
are called respective- 
ly the hour circle and 
the declination circle. 
The telescope is at- 
tached directly to 
the declination axis. 
When the telescope is 
urned on its polar axis. 



20 



ASTRONOMY. 



the line of sight will describe a diurnal circle ; so that, when 
the tube is once directed to a star, it can be made to fol- 
low the star by simply turning the telescope on its polar axis. 

In the case of large instruments of this class, the polar 
axis is usually turned by clock-work at the rate at which the 
heavens rotate : so that, when the telescope has once been 
pointed to a planet or other heavenly body, it will continue to 
follow the body and keep it steadily in the field of view without 
further trouble on the part of the observer. 

The great Washington Equatorial is shown in Fig. 22. Its 
object-glass is 26 inches in diameter, and its focal length is 
32^ feet. It was constructed by Alvan Clark & Sons of Cam- 
bridge. Mass. It is one of the three largest refracting tele- 
scopes at present in use. The Xewall refractor at Gateshead, 
Eng., has an objective 25 inches in diameter, and a focal length 
of 29 feet. The great refractor at Vienna has an objective 
27 inches in diameter. There are several large refractors now 
in process of construction. 

19. The Wire Micrometer. — Large arcs in the heavens 
are measured by means of the graduated circles attached to 

the axes of the 
telescopes ; but 
small arcs within 
the field of view 
of the telescope 
are measured by means of instruments called micrometers, 
mounted in the focus of the telescope. One of the most 
convenient of these micrometers is that known as the wire 
micrometer, and shown in Fig. 23. 

The frame A A covers two slides, C and D. These slides 
are moved by the screws F and G. The wires F and B 
are stretched across the ends of the slides so as to be 
parallel to each other. On turning the screws F and G 
one way, these wires are carried apart ; and on turning them 
the other way they are brought together again. Sometimes 
two parallel wires, x and y, shown in the diagram at the 




ASTRONOMY. 21 

right, are stretched across the frame at right angles to the 
wires E, B. We may call the wires x and y the longitudi- 
nal wires of the micrometer, and E and B the transverse 
wires. Many instruments have only one longitudinal wire, 
which is stretched across the middle of the focus. The 
longitudinal wires are just in front of the transverse wires. 
but do not touch them. 

To find the distance between any two points in the field 
of view with a micrometer, with a single longitudinal wire, 
turn the frame till the longitudinal wire passes through the 
two points : then set the wires E and B one on each 
point, turn one of the screws, known as the micrometer 
screw, till the two wires are brought together, and note the 
number of times the screw is turned. Having previously 
ascertained over what arc one turn of the screw will move 
the wire, the number of turns will enable us to find the 
length of the arc between the two points. 

The threads of the micrometer screw are cut with great 
accuracy ; and the screw is provided with a large head, which 
is divided into a hundred or more equal parts. 

These divisions, by means of a fixed pointer, enable us to 
ascertain what fraction of a turn the screw has made over 
and above its complete revolutions. 

20. Reflecting Telescopes. — It is possible to construct 
mirrors of much larger size than lenses : hence reflecting 
telescopes have an advantage over refracting telescopes as 
regards size of aperture and of light-gathering power. They 
are, however, inferior as regards definition ; and, in order 
to prevent flexure, it is necessary to give the speculum, or 
mirror, a massiveness which makes the telescope unwieldy. 
It -is also necessary frequently to repolish the speculum : 
and this is an operation of great delicacy, as the slightest 
change in the form of the surface impairs the definition of 
the image. These defects have been remedied, to a certain 
extent, by the introduction of silver-on-glass mirrors ; that is, 



22 



ASTRONOMY. 



glass mirrors covered in front with a thin coating of silver. 
Glass is only one-third as heavy as speculum-metal, and sil- 
ver is much superior to that metal in reflecting power ; and 
when the silver becomes tarnished, it can be removed and 
renewed without danger of changing the form of the glass. 

The Herschelian Reflector. — In this form of telescope the 
mirror is slightly tipped, so that the image, instead of being 
formed in the centre of the tube, is formed near one side 
of it, as in Fig. 24. The observer can then view it with- 
out putting his head inside the tube, and therefore without 
cutting off any material portion of the light. In observa- 
tion, he must stand at the upper or outer end of the tube, 
and look into it, his back being turned towards the object. 
From his looking 
directly into the 
mirror, it is also 
sometimes called 
the front -view 
telescope. The 

great disadvan- f 71 ' 1 ! 

tage of this ar- Fi - 24 - 

rangement is, that the rays cannot be brought to an exact 
focus when they are thrown so far to one side of the axis, 
and the injury to the definition is so great that the front- 
view plan is now entirely abandoned. 

The Newtonian Reflector. — The plan proposed by Sir 
Isaac Newton was to place a small plane mirror just inside 
the focus, inclined to the telescope at an angle of 45 °, so 
as to throw the rays to the side of the tube, where they 
come to a focus, and form the image. An opening is made 
in the side of the tube, just below where the image is 
formed ; and in this opening the eye-piece is inserted. The 
small mirror cuts off some of the light, but not enough to 
be a serious defect. An improvement which lessens this 
defect has been made by Professor Henry Draper. The 




ASTRONOMY. 



23 



inclined mirror is replaced by a small rectangular prism 
(Fig. 25), by reflection from which the image is formed 
very near the prism. A pair of lenses are then inserted in 
the course of the rays, by which a second image is formed 
at the opening in the side of the tube ; and this second 
image is viewed by an ordinary eye-piece. 

The Gregorian Reflector. — This is a form proposed by 




Fig. 25. 



James Gregory, who probably preceded Newton as an in- 
ventor of the reflecting telescope. Behind the focus, F 
(Fig. 26), a small concave mirror, R, is placed, by which 
the light is reflected back again down the tube. The larger 
mirror, M, has an opening through its centre ; and the small 
mirror, R, is so adjusted as to form a second image of the 
object in this opening. This image is then viewed by an 
eye-piece which is screwed into the opening. 




Fig. 26. 

The Cassegrainian Reflector. — In principle this is the 
same with the Gregorian ; but the small mirror, R, is con- 
vex, and is placed inside the focus, F, so that the rays are 
reflected from it before reaching the focus, and no image 
is formed until they reach the opening in the large mirror. 
This form has an advantage over the Gregorian, in that the 



24 



ASTRONOMY.. 



telescope may be made shorter, and the small mirror can 
be more easily shaped to the required figure. It has, there- 
fore, entirely superseded the original Gregorian form. 




Optically these forms of telescope are inferior to the 
Newtonian ; but the latter is subject to the inconvenience, 
that the observer must be stationed at the upper end of 
the telescope, where he looks into an eye-piece screwed 
into the side of the tube. 



ASTRONOMY'. 25 

( )n the other hand, the .Cassegrainian Telescope is pointed 
directly at the object to be viewed, like a refractor ; and the 
observer stands at the lower end, and looks in at the open- 
ing through the large mirror. This is, therefore, the most 
convenient form of all in management. 




Fig. 28. 

The largest reflecting telescope vet constructed is that of 
Lord Rosse, at Parsonstown, Ireland. Its speculum is 6 feet 
in diameter, and its focal length 55 feet. It is commonly used 
as a Newtonian. This telescope is shown in Fig. 27. 

The great telescope of the Melbourne Observatory, Austra- 
lia, is a Cassegranian reflector. Its speculum is 4 feet in 



26 



ASTRONOMY. 



diameter, and its focal length is 32 feet. It is shown in 
Fig. 28. 

The great reflector of the Paris Observatory is a Newtonian 




Fig. 29. 

reflector. Its mirror of silvered glass is 4 feet in diameter, 

and its focal length is 23 feet. This telescope is shown in 
Fig. 29. 

21. The Sun's Motion among the Stars, — If we notice 



ASTRONOMY. 



2J 



the stars at the same hour night after night, we shall find 
that the constellations are steadily advancing towards the 
west. New constellations are continually appearing in the 
east, and old ones disappearing in the west. This continual 
advancing of the heavens towards the west is due to the fact 
that the sun's place among the stars is continually moving 
towards the east. The sun completes the circuit of the 
heavens in a year, and is therefore moving eastward at the 
rate of about a degree a day. 

This motion of the sun's place among the stars is due 
to the revolution of 
the earth around the 
sun, and not to any 
real motion of the 
sun. In Fig. 30 sup- 
pose the inner circle 
10 represent the orbit 
of the earth around 
the sun, and the outer 
circle to represent the 
celestial sphere. When 
the earth is at E, the 
sun's place on the 
celestial sphere is at 
S'. As the earth moves in the direction E F, the sun's 
place on the celestial sphere must move in the direction 
S' T : hence the revolution of the earth around the sun 
would cause the sun's place among the stars to move around 
the heavens in the same direction that the earth is moving 
around the sun. 

22. The Ecliptic. — The circle described by the sun in" 
its apparent motion around the heavens is called the ecliptic. 
The plane of this circle passes through the centre of the 
earth, and therefore through the centre of the celestial 
sphere : the earth being so small, compared with the celes- 




28 



ASTRONOMY. 




tial sphere, that it practically makes no difference whether 
we consider a point on its surface, or one at its centre, as 
the centre of the celestial sphere. The ecliptic is, therefore. 

a great circle. 

The earth's orbit lies in the plane of the ecliptic ; but it 
extends only an inappreciable distance from the sun towards 

the celestial sphere. 

23. The Obliquity of the 
Ecliptic. — The ecliptic is in- 
clined to the celestial equator 
by an angle of about 23V" '. 
This inclination is called the 
obliquity of the ecliptic. The 
obliquity of the ecliptic is due 
to the deviation of the earth's axis from a perpendicular to 
the plane of its orbit. The axis of a* rotating body tends 
to maintain the same direction ; and, as the earth revolves 
around the sun. its axis points all the time in nearly the 
same direction. The earth's axis deviates about 23J from 
the perpendicular to its orbit ; and, as the earth's equator is 
at right angles to its axis, it 
will deviate about 23-J- from 
the' plane of the ecliptic. 
The celestial equator has the 
same direction as the terres- 
trial equator, since the axis 
of the heavens has the same 
direction as the axis of the 
earth. 

Suppose the globe at the centre of the tub (Fig. 31) to 
represent the sun, and the smaller globes to represent the 
earth in various positions in its orbit. The surface of the 
water will then represent the plane of the ecliptic, and 
the rod projecting from the top of the earth will represent 
the earth's axis, which is seen to point all the time in the 







Fig. 



ASTRONOMY. 



2 9 




The points at which 



same direction, or to lean the same way. The leaning of 
the axis from the perpendicular to the surface of the water 
would cause the earth's equator to be inclined the same 
amount to the surface of the water, half of the equator being- 
above, and half of it below, 
the surface. Were the axis 
of the earth perpendicular 
to the surface of the water, 
the earth's equator would 
coincide with the surface, as 
is evident from Fig. 32. 

24. The Equinoxes and 
Solstices. — The ecliptic and 
celestial equator, being great 
circles, bisect each other. 
Half of the ecliptic is north, 
and half of it is south, of the equator, 
the two circles cross are called the equinoxes. The one at 
which the sun crosses the equator from south to north is 
called the vernal equinox, and the one at which it crosses 

from north to south the 
autumnal equinox. The 
points on the ecliptic mid- 
way between the equinoxes 
are called the solstices. The 
N one north of the equator is 
called the summer sglstice, 
and the one south of the 
equator the winter solstice. 
In Fig. 33, E Q is the celes- 
tial equator, Ec E r c f the 
ecliptic, V the vernal equinox, A the autumnal equinox, E c 
the winter solstice, and E' c' the summer solstice. 

25. The Inclination of the Ecliptic to the Horizon. — 
Since the celestial equator is perpendicular to the axis of 




TE& 



30 ASTRONOMY. 

the heavens, it makes the same angle with it on every side : 
hence, at any place, the equator makes always the same 
angle with the horizon, whatever part of it is above the hori- 
zon. But, as the ecliptic is oblique to the equator, it makes 
different angles with the celestial axis on different sides ; 
and hence, at any place, the angle which the ecliptic makes 
with the horizon varies according to the part which is above 
the 'horizon. The two extreme angles for a place more than 
23^° north of the equator are shown in Figs. 34 and 35. 

The least angle is formed when the vernal equinox is on 
the eastern horizon, the autumnal on the western horizon, 
and the winter solstice on the meridian, as in Fig. 34. The 

angle which the ecliptic then 
makes with the horizon is 
equal to the elevation of the 
equinoctial minus 23V. In 
the latitude of New York this 
angle = 49 - 23-J = 25I . 
The greatest angle is 
formed when the autumnal 
equinox is on the eastern 
horizon, the vernal on the 
western horizon, and the 
summer solstice is on the meridian (Fig. 35). The angle 
between the ecliptic and the horizon is then equal to the 
elevation of the equinoctial plus 23+ . In the latitude of 
New York this angle = 49 + 23^° = 72^°. 

Of course the equinoxes, the solstices, and all other points 
on the ecliptic, describe diurnal circles, like every other 
point in the heavens : hence, in our latitude, these points 
rise and set every day. 

26. Celestial Latitude and Longitude. — Celestial lati- 
tude is distance measured north or south from the ecliptic ; 
and celestial lo7igitude is distance measured on the ecliptic 
eastward from the vernal equinox, or the first point of 




ASTRONOMY. 



31 



Aries. Great circles perpendicular to the ecliptic are called 
celestial meridians. These circles all pass through the 
poles of the ecliptic, which are some 23^° from the poles of 
the equinoctial. The latitude of a heavenly body is meas- 
ured by the arc of a celestial meridian included between 
the body and the ecliptic. The longitude of a heavenly 
body is measured by the arc of the ecliptic included be- 
tween the first point of Aries and the meridian which passes 
through the body. There are, of course, always two arcs 
included between the first point of Aries and the meridian? 
— one on the east, and the other on the west, of the first 
point of Aries. The one on the east is always taken as 
the measure of the longitude. 

27. The Precession of the 
Equinoxes. — The equinoc- 
tial points have a slow west- 
ward motion along the eclip- 
tic. This motion is at the 
rate of about 50" a year, and 
would cause the equinoxes 
to make a complete circuit 
of the heavens in a period 
of about twenty-six thousand 
years. It is called the precession of the equinoxes. This 
westward motion of the equinoxes is due to the fact that 
the axis of the earth has a slow gyratory motion, like the 
handle of a spinning-top which has begun to wabble a little. 
This gyratory motion causes the axis of the heavens to 
describe a cone in about twenty-six thousand years, and the 
pole of the heavens to describe a circle about the pole of 
the ecliptic in the same time. The radius of this circle 
is 2 3-I . 

28. Illustration of Precession. — The precession of the 
equinoxes may be illustrated by means of the apparatus shown 
in Fig. 36. The horizontal and stationary ring E C represents 




Fig. 36. 



2 2 ASTRONOMY. 

the ecliptic; the oblique ring E Q represents the equator: 
V and A represent the equinoctial point, and E and C the 
solstitial points: B represents the pole of the ecliptic, P the 
pole of the equator, and P O the celestial axis. The ring E' Q 
is supported on a pivot at Oj and the rod B P, which connects 
B and P, is jointed at each end so as to admit of the move- 
ment of P and B. 

On carrying P around B, we shall see that E Q will always 
preserve the same obliquity to E C, and that the points Fancl A 
will move around the circle E C. The same will also be true 
of the points E and C. 

29. Effects of Precession. — One effect of precession, as 
has already been stated, is the revolution of the pole of the 
heavens around the pole of the ecliptic in a period of about 
twenty-six thousand years. The circle described by the 
pole of the heavens, and the position of the pole at various 
dates, are shown in Fig. 37, where o indicates the position 
of the pole at the birth of Christ. The numbers round 
the circle to the left of o are dates A.D., and those to the 
right of o are dates B.C. It will be seen that the star at 
the end of the Little Bear's tail, which is now near the 
north pole, will be exactly at the pole about the year 2000. 
It will then recede farther and farther from the pole till the 
year 15000 A.D., when it will be about forty-seven degrees 
away from the pole. It will be noticed that one of the stars 
of the Dragon was the pole star about 2800 years B.C. 
There are reasons to suppose that this was about the time 
of the building of the Great Pyramid. 

A second effect of precession is the shifting of the signs 
along the zodiac. The zodiac is a belt of the heavens 
along the ecliptic, extending eight degrees from it on each 
side. This belt is occupied by twelve constellations, known 
as the zodiacal constellations. They are Aries, Taurus. 
Gemini. Cancer, Leo, Virgo. Libra. Scorpio, Sagittarius, 
Capncornus. Aquarius, and Pisces. The zodiac is also 



ASTRONOMY. 33 

divided into twelve equal parts of thirty degrees each, called 
signs. These signs have the same names as the twelve 
zodiacal constellations, and when they were first named, 
each sign occupied the same part of the zodiac as the cor- 
responding constellation ; that is to say, the sign Aries was 
in the constellation Aries, and the sign Taurus in the con- 



* : * \ A 




eJ »enl of ^ * VT;*Y 






^: \ rli W 



Little! Bear / \ ; I 



^ 



e \ y\ f c the I o * Uo 0<> \! V 

<** &' r"& "V 3*" v'"' / io °o 




Fig. 37- 

stellation Taurus, etc. Now the signs are always reckoned 
as beginning at the vernal equinox, which is continually 
shifting along the ecliptic ; so that the signs are continually 
moving along the zodiac, while the constellations remain 
stationary : hence it has come about that the first point of 
Aries (the sign) is no longer in the constellation Aries, but 
in Pisces. 



34 ASTRONOMY. 

Fig. 38 shows the position of the vernal equinox 2170 
B.C. It was then in Taurus, just south of the Pleiades. 
It has since moved from Taurus, through Aries, and into 
Pisces, as shown in Fig. 39. 




Fig. 38. 

Since celestial longitude and right ascension are both 
measured from the first point of Aries, the longitude and 
right ascension of the stars are slowly changing from vear 




Fig. 39- 



to year. It will be seen, from Figs. 3 S and 39, that the 
declination is also slowly changing. 

30. Nutation. — The gyratory motion of the earth's axis 
is not perfectly regular and uniform. The earth's axis has 



ASTRONOMY. 



35 




a slight tremulous motion, oscillating to and fro through a 
short distance once in about nineteen years. This tremu- 
lous motion of- the axis causes the pole of the heavens to 
describe an undulating curve, as shown in Fig. 40, and 
gives a slight unevenness to the motion of the equinoxes 
along the ecliptic. This nodding motion of the axis is 
called nutation. 

31. Refraction. — When a ray of light 
from one of the heavenly bodies enters the 
earth's atmosphere obliquely, it will be bent 
towards a perpendicular to the surface of 
the atmosphere, since it will be entering a 
denser medium. As the ray traverses the Flg * 4 °- 

atmosphere, it will be continually passing into denser and 
denser layers, and will therefore be bent more and more 
towards the perpendicular. This bending of the ray is 
shown in Fig. 41. A ray which started from A would enter 
the eye at C. as if it came from /.* hence a star at A would 
appear to be at I. 

Atmospheric refraction displaces all the heavenly bodies 

from the horizon towards the 
zenith. This is evident from 
Fig. 42. OD is the horizon, 
and Z the zenith, of an observer 
at O. Refraction would make 
a star at Q appear at P : in 
other words, it would displace 
it towards the zenith. A star in 
the zenith is not displaced by 
refraction, since the rays which reach the eye from it traverse 
the atmosphere vertically. The farther a star is from the 
zenith, the more it is displaced by refraction, since the 
greater is the obliquity with which the rays from it enter 
the atmosphere. 

At the horizon the displacement by refraction is about 




36 



ASTRONOMY, 



half a degree : but it varies considerably with the state of 
the atmosphere. Refraction causes a heavenly bodv to 




Fig. 42. 

appear above the horizon longer than it really is above it, 
since it makes it appear to be on the horizon when it is 
reallv half a degree below it. 





Fig- 43- 

The increase of refraction towards the horizon often 
makes the sun. when near the horizon, appear distorted, 



ASTRONOMY. 



37 



the lower limb of the sun being raised more than the upper 
limb. This distortion is shown in Fig. 43. The vertical 
diameter of the sun appears to be considerably less than 
the horizontal diameter. 

32. Parallax. — Parallax is the displacement of an 
object caused by a change in the point of view from which 
it is seen. Thus in Fig. 44, the top of the tower S would 
be seen projected against the sky at a by an observer at A, 
and at b by an observer at B. In passing from A to B. 
the top of the tower is displaced from a to b, or by the 




Fig. 44. 



angle a S Ik This angle is called the parallax of £. as seen 
from B instead of A. 

The geocentric parallax of a heavenly body is its dis- 
placement caused by its being seen from the surface of the 
earth, instead of from the centre of the earth. In Fig. 45. 
R is the centre of the earth, and O the point of observation 
on the surface of the earth. Stars at S, S\ and S"> would, 
from the centre of the earth, appear at Q, Q'. and Q" ; 
while from the point O on the surface of the earth, these 
same stars would appear at P, P' , and P" , being displaced 



38 



ASTRONOMY. 



from their position, as seen from the centre of the earth, by 
the angles QSP, Q'S'P\ and Q"S"P". It will be seen 
that parallax displaces a body from the zenith towards the 
horizon, and that the parallax of a body is greatest when it 
is on the horizon. The parallax of a heavenly body when 
on the horizon is called its horizontal parallax. A body 
in the zenith is not displaced by parallax, since it would 
be seen in the same direction from both the centre and 

the surface of the 
earth. 

The parallax of 
a body at S'" is 
Q'"S"'P, which is 
seen to be greater 
r than QSP; that 
is to say, the par- 
B allax of a heaven- 
ly body increases 
with its nearness 
to the earth. The 
distance and parallax of a body are so related, that, either 
being known, the other may be computed. 

33. Aberration. — Aberration is a slight displacement of a 
star, owing to an apparent change in the direction of the rays 
of light which proceed from it. caused by the motion of the 
earth in its orbit. If we walk rapidly in any direction in the 
rain, when the drops are falling vertically, they will appear to 
come into our faces from the direction in which we are walking. 
Our own motion has apparently changed the direction in which 
the drops are falling. 

In Fig. 46 let A be a gun of a battery, from which a shot 
is fired at a ship, D E, that is passing.' Let A B C be the 
course of the shot. The shot enters the ship's side at B, and 
passes out at the other side at Cj but in the mean time the 
ship has moved from E to e. and the part B, where the shot 
entered, has been carried to b % If a person on board the ship 




Fig. 45- 



ASTRONOMY. 



39 



0- 



7)iz 

J 




could see the ball as it crossed the ship, he would see it cross 
in the diagonal line b Cj and he would at once say that the 
cannon was in the direction of C b. If the ship were moving 
in the opposite direction, he would 
say that the cannon was just as far 
the other side of its true position. 

Now, we see a star in the direction 
in which the light coming from the 
star appears to be moving. When 
we examine a star with a telescope, 
we are in the same condition as the 
person who on shipboard saw the 
cannon-ball cross the ship. The tele- 
scope is carried along by the earth lg ' 4 °' 
at the rate of eighteen miles a second: hence the light will 
appear to pass through the tube in a slightly different direction 
from that in which it is really moving: just as the cannon-ball 
appears to pass through the ship in a different direc- 
tion from that in which it is really moving. Thus in 
Fig. 47. a ray of light coming in the direction SOT 
would appear to traverse the tube O T of a telescope, 
moving in the direction of the arrow, as if it were 
coming in the direction S' O. 

As light moves with enormous velocity, it passes 
through the tube so quickly, that it is apparently 
changed from its true direction only by a very slight 
angle: but it is sufficient to displace the star. This 
apparent change in the direction of light caused by 
the motion of the earth is called aberration of light. 

34. The Planets. — On watching the stars atten- 
tively night after night, it will be found, that while 
the majority of them appear fixed on the surface 
of the celestial sphere, so as to maintain their rela- 
tive positions, there are a few that wander about 
among the stars, alternately advancing towards the 
east, halting, and retrograding towards the west. These wan- 
dering stars are called planets. 

Their motions appear quite irregular ; but, on the whole. 



9 

Fig. 47- 



40 ASTRONOMY. 

their eastward motion is in excess of their westward, and in 
a longer or shorter time they all complete the circuit of the 
heavens. In almost every instance, their paths are found to 
lie wholly in the belt of the zodiac. 



Fig. 48 shows a portion of the apparent path of one of 
the planets. 



II. 

THE SOLAR SYSTEM. 



I. THEORY OF THE SOLAR SYSTEM. 

35. Members of the Solar System. — The solar system 
is composed of the sun, planets, moons, eomets, and meteors. 
Five planets, besides the earth, are readily distinguished by 
the naked eye, and were known to the ancients : these are 
Mercury, Venus, Mars, Jupiter, and Saturn. These, with 
the sun and moon, made up the seven planets of the ancients, 
from which the seven days of the week were named. 

The Ptolemaic System. 

36. The Crystalline Spheres. — We have seen that all the 
heavenly bodies appear to be situated on the surface of the 
celestial sphere. The ancients assumed that the stars were 
really fixed on the surface of a crystalline sphere, and that 
they were carried around the earth daily by the rotation of 
this sphere. They had, however, learned to distinguish the 
planets from the stars, and they had come to the conclusion 
that some of the planets were nearer the earth than others, 
and that all of them were nearer the earth than the stars 
are. This led them to imagine that the heavens were com- 
posed of a number of crystalline spheres, one above another, 
each carrying one of the planets, and all revolving around 
the earth from east to west, but at different rates. This 
structure of the heavens is shown in section in Fig. 49. 

41 



42 



ASTRONOMY. 



37. Cycles and Epicycles. —The ancients had also noticed 
that, while all the planets move around the heavens from 
west to east, their motion is not one of uniform advance- 
ment. Mercury and Venus appear to oscillate to and fro 
across the sun, while Jupiter and Saturn appear to oscillate 
to and fro across a centre which is moving around the 
earth, so as to describe a series of loops, as shown in 
Fig. 50. 




Fig. 49- 

The ancients assumed that the planets moved in exact 
circles, and, in fact, that all motion in the heavens was 
circular, the circle being the simplest and most perfect 
curve. To account for the loops described by the planets, 
they imagined that each planet revolved in a circle around a 
centre, which, in turn, revolved in a circle around the earth. 
The circle described by this centre around the earth they 
called the cycle, and the circle described by the planet 
around this centre they called the epicycle. 



ASTRONOMY. 



43 



38. The Eccentric. — The ancients assumed that the 
planets moved at a uniform rate in describing the epicycle, 
and also the centre in describing the cycle. They had, 
however, discovered that the planets advance eastward more 
rapidly in some parts of their orbits than in others. To 




Fig. 5°- 

account for this they assumed that the cycles described by 
the centre, around which the planets revolved, were eccen- 
tric ; that is to say. that the earth was not at the centre 
of the cycle, but some distance away from it. as shown 
in Fig. 51. E is the position of the earth, and C is the 



44 



ASTRONOMY, 




centre of the cycle. The lines from E are drawn so as to 
intercept equal arcs of the cycle. It will be seen at once 
that the angle between any pair of lines is greatest at P, and 

least at A; so that, were a 
planet moving at the same rate 
at P and A, it would seem to 
be moving much faster at P. 
The point P of the planet's 
cycle was called its perigee, and 
the point A its apogee. 

As the apparent motion of 
the planets became more ac- 
curately known, it was found 
necessary to make the system 
of cycles, epicycles, and eccen- 
trics exceedingly complicated to represent that motion. 

The Copernican System. 

39. Copernicus. — Copernicus simplified the Ptolemaic 
system greatly by assuming that the earth and all the planets 
revolved about the sun as a centre. He, however, still main- 
tained that all motion in the heavens was circular, and hence 
he could not rid his system entirely of cycles and epicycles. 

Tycho Brahe's System. 

40. Tycho Brake. — Tycho Brahe was the greatest of the 
early astronomical observers. He, however, rejected the sys- 
tem of Copernicus, and adopted one of his own, which was 
much more complicated. He held that all the planets but 
the earth revolved around the sun, while the sun and moon 
revolved around the earth. This system is shown in Fig. 52. 



Kepler's System. 

41. Kepler. — While Tycho Brahe devoted his life to the 
observation of the planets. Kepler gave his to the study 



ASTRONOMY. 



45 



ot Tycho's observations, for the purpose of discovering the 
true laws of planetary motion. He banished the compli- 
cated system of cycles, epicycles, and eccentrics forever from 
the heavens, and discovered the three laws of planetary 
motion which have rendered his name immortal. 

42. The Ellipse. — An ellipse is a closed curve which has 
two points within it, the sum of whose distances from every 
point on the curve is the same. These two points are called 
ihefoci of the ellipse. 



^y^ffl^ff^W^ 




Fig. 52. 

One method of describing an ellipse is shown in Fig. 53. 
Two tacks, F and F\ are stuck into a piece of paper, and 
to these are fastened the two ends of a string which is longer 
than the distance between the tacks. A pencil is then 
placed against the string, and carried around, as shown in 
the figure. The curve described by the pencil is an ellipse. 
The two points F and F' are the foci of the ellipse : the 
sum of the distances of these two points from every point 
on the curve is equal to the length of the string. When 
half of the ellipse has been described, the pencil must be 



4 6 



ASTRONOMY. 



held against the other side of the string in the same way. 
and carried around as before. 

The point O, half way between F and F' '. is called the 




centre of the ellipse ; A A' is the major axis of the ellipse, 
and CD is the minor axis. 

43. The Fccentricity of the Fllipse. — The ratio of the 
distance between the two foci to the major axis of the 
ellipse is called the eccentricity of the ellipse. The greater 

the distance between the two 
foci, compared with the major 
axis of the ellipse, the greater is 
the eccentricity of the ellipse ; 
and the less the distance be- 
tween the foci, compared with 
the length of the major axis, 
the less the eccentricity of the 
ellipse. The ellipse of Fig. 54 
has an eccentricity of \. This 
ellipse scarcely differs in appear- 
ance from a circle. The ellipse of Fig. 55 has an eccen- 
tricity of i and that of Fig. 56 an eccentricity of J. 

44. Kepler's First Law. — Kepler first discovered that 
all the planets move from west to east in ellipses which have 




F ig- 54. 



ASTRONOMY. 



47 




Fig. 55- 



the sun as a common focus. This law of planetary motion 
is known as Kepler's Fust Law. The planets appear to 
describe loops, because we view them from a moving point. 

The ellipses described by the planets differ in eccentricity ; 
and, though they all have one focus at the sun, their major 
axes have different directions. The eccentricity of the plan- 
etary orbits is comparatively 
small. The ellipse of Fig. 54 
has seven times the eccentricity 
of the earth's orbit, and twice 
that of the orbit of any of the 
larger planets except Mercury \ 
and its eccentricity is more 
than half of that of the orbit 
of Mercury. Owing to their 
small eccentricity, the orbits of 
the planets are usually represented by circles in astronomi- 
cal diagrams. 

45. Kepler's Second Law. — Kepler next discovered that 
a planet's rate of motion in the various parts of its orbit 
is such that a line drawn from the planet to the sun would 
always sweep over equal areas in equal times. Thus, in 
Fig. 57, suppose the planet would move from P\o P l in the 

same time that it would move 
from P 2 to P\ or from P* to 
P s ; then the dark spaces, which 
would be swept over by a line 
joining the sun and the planet, 
in these equal times, would all 
be equal. 

A line drawn from the sun to a planet is called the 
radius vector of the planet. The radius vector of a planet 
is shortest when the planet is nearest the sun, or at perihe- 
lion, and longest when the planet is farthest from the sun, 
or at aphelion : hence, in order to have the areas equal, it 




Fig. 56. 



4 <S ASTRONOMY. 

follows that a planet must move fastest when at perihelion, 
and slowest at aphelion. 

Kepler's See and Law of planetary motion is usually 
stated as follows : The radius vector of a planet describes 
equal areas in equal times in every part of the planet's 
orbit. 

46. Kepler's Third Law. — Kepler finally discovered that 

the periodic times 
of the planets bear 
the following rela- 
tion to the distan- 
ces of the planets 
i^ s from the sun : The 
squares of the peri- 
odic times of the 
planets are to each 
other as the cubes 
of their mean dis- 
tances from the sun. This is known as Kepler's Third Law 
of planetary motion. By periodic time is meant the time it 
takes a planet to revolve around the sun. 

These three laws of Kepler's are the foundation of mod- 
ern physical astronomy. 

The Newtonian System. 

47. Newton's Discovery.— -.Newton followed Kepler, and 
by means of his three laws of planetary motion made his 
own immortal discovery of the law of gravitation. This 
law is as follows : Every portion of matter in the universe 
attracts every other portion, with a force varying directly as 
the product of the masses acted upon, and inversely as the 
square of the distances between them. 

48. The Conic Sections. — The conic sections are the 
figures formed by the various plane sections of a right cone. 
There are four classes of figures formed bv these sections, 







ASTRONOMY. 



49 



according to the angle which the plane of the section 
makes with the axis of the cone. 

OPQ, Fig. 58, is a right cone, and ON is its axis. 
Any section, AB, of this cone, whose plane is perpendicular 
to the axis of the cone, is a circle. 

Any section, CD, of this cone, whose plane is oblique 
to the axis, but forms with it an angle greater than NOP, 
is an ellipse. The less the angle which the plane of the 
section makes with the 
axis, the more elongated 
is the ellipse. 

Any section, £ F, of 
this cone, whose plane 
makes with the axis an 
angle equal to NOP, is 
a parabola. It will be 
seen, that, by changing the 
obliquity of the plane CD 
to the axis NO, we may 
pass uninterruptedly from 
the circle through ellipses 
of greater and greater 
elongation to the parabola. t 

Any section, GH, of 
this cone, whose plane 
makes with the axis ON 
an angle less than NOP, is a hyperbola. 

It will be seen from Fig. 59, in which comparative views 
of the four conic sections are given, that the circle and 
the ellipse are closed curves, or curves which return into 
themselves. The parabola and the hyperbola are, on the 
contrary, open curves, or curves which do not return into 
themselves. 

49. A Revolving Body is continually Falling towards its 
Centre of Revolution. — In Fig. 60 let M represent the moon, 




50 



ASTRONOMY. 



and E the earth around which the moon is revolving in the 
direction MN. It will be seen that the moon, in moving from 
J/ to yV, falls towards the earth a distance equal to mN. It 
is kept from falling into the earth by its orbital motion. 

The fact that a 
body might be pro- 
jected forward fast 
enough to keep it 
from falling into the 
earth is evident from 
Fig. 61. AB repre- 
sents the level sur- 
face of the ocean, 
C a mountain from 
the summit of which 
a cannon-ball is sup- 
posed to be fired in 
the direction C E. 
A D is a line parallel 
with CE; DB is a 
line equal to the dis- 
tance between the two 
Flg - 59 ' parallel lines A D and 

CE. This distance is equal to that over which gravity would 
pull a ball towards the centre of the earth in a minute. Xo 
matter, then, with what velocity the ball C is tired, at the end 
of a minute it will be somewhere on the line A D. Suppose 
it were fired fast enough to reach the point D in a minute: 
it would be on the line A D at the end of the 
minute, but still just as far from the surface of 
the water as when it started. It will be seen, 
that, although it has all the while been falling 
towards the earth, it has all the while kept at 
exactly the same distance from the surface. 
The same thing would of course be true dur- 
ing each succeeding minute, till the ball came 
round to C again, and the ball would continue to revolve in a 
circle around the earth. 

50. The Form of a Body's Orbit depends upon the Rate of 





ASTRONOMY. 



Si 



its Forward Motion, — If the ball C were fired fast enough to 
reach the line AD beyond the point D, it would be farther 
from the surface at the end of the second than when it 
started. Its orbit would no longer be circular, but ellipti- 




cal. If the speed of projection were gradually augmented, 
the orbit would become a more and more elongated ellipse. 
At a certain rate of projection, the orbit would become a 
parabola ; at a still greater rate, 
a hyperbola. 

51. The Moon held in her 
Orbit by Gravity. — Newton com- 
pared the distance m N that the 
moon is drawn to the earth in 
a given time, with the distance 
a body near the surface of the 
tartli would be pulled toward 
the earth in the same time : and 
he found that these distances 
are to each other inversely as 
the squares of the distances of 
the two bodies from the centre 
of the earth. He therefore con- 
cluded that the moon is drawn 

to the earth by gravity, and that the intensity of gravity 
decreases as the square of the distance increases. 

52. Any Body whose Orbit is a Conic Section, and which 
moves according to Kepler 1 s Second Law. is acted upon by a 




52 



ASTRONOMY. 



Force varying inversely as the Square of the Distance. — New- 
ton compared the distance which any body, 
moving in an ellipse, according to Kepler's 
Second Law, is drawn towards the sun in the 
same time in different parts of its orbit. He 
found these distances in all cases to vary 
inversely as the square of the distance of 
the planet from the sun. Thus, in Fig. 62, 
suppose a planet would move from K to B 
in the same time that it would move from k 
to b in another part of its orbit. In the first 
instance the planet would be drawn towards 
the sun the distance A B, and in the second 
instance the distance ab. Newton found that 
AB : ab- SK* : Sk*. He also found that 
the same would be true when the body moved 
in a parabola or a hyperbola: hence he con- 
cluded that every body that moves around the 
sun in an ellipse, a parabola, or a hyperbola, 
is moving under the influence of gravity. 

53. The Force that draws the Different 
Sun Varies inversely as the Squares of the 

Distances of the Planets from the Sun. — Newton compared 

the distances jK and eF, 

over which two planets are 

drawn towards the sun in 

the same time, and found 

these distances to vary 

inversely as the squares 

of the distances of the 

planets from the sun : 

hence he concluded that 

all the planets ai'e held 

in their orbits by gravity. 

He also showed that this 

would be true of any two 

bodies that were revolving 

around the sun's centre, 

according to Kepler's Third Law 




Fig. 63. 
Planets to the 




Fig. 64. 



ASTRONOMY. 53 

54. The Copernican System.- — The theory of the solar 
system which originated with Copernicus, and which was 
developed and completed by Kepler and Newton, is com- 
monly known as the Copernican System. This system is 
shown in Fig. 64. 

II. THE SUN AND PLANETS. 
I. THE EARTH. 

Form and Size. 

55. Form of the Earth. — In ordinary language the 
term horizon denotes the line that bounds the portion of 
the earth's surface that is visible at any point. 

(1) It is well known that the horizon of a plain presents 
the form of a circle surrounding the observer. If the 
latter moves, the circle moves also ; but its form remains the 
same, and is modified only when mountains or other obsta- 
cles limit the view. Out at sea, the circular form of the 
horizon is still more decided, and changes only near the 
coasts, the outline of which breaks the regularity. 

Here, then, we obtain a first notion of the rotundity of 
the earth, since a sphere is the only body which is presented 
always to us under the form of a circle, from whatever point 
on its surface it is viewed. 

(2) Moreover, it cannot be maintained that the horizon 
is the vanishing point of distinct vision, and that it is this 
which causes the appearance of a circular boundary, because 
the horizon is enlarged when we mount above the surface 
of the plain. This will be evident from Fig. 65, in which a 
mountain is depicted in the middle of a plain, whose uni- 
form curvature is that of a sphere. From the foot of the 
mountain the spectator will have but a very limited horizon. 
Let him ascend half way, his visual radius extends, is inclined 
below the first horizon, and reveals a more extended circu- 



54 



ASTRONOMY. 



lar area. At the summit of the mountain the horizon still 
increases ; and, if the atmosphere is pure, the spectator will 




Fig. 6s 



see numerous objects where from the lower stations the sky 
alone was visible. 




Fig. 



This extension of the horizon would be inexplicable if 
the earth had the form of an extended plane. 



ASTRONOMY. 



55 



(3) The curvature of the surface of the sea manifests 
itself in a still more striking manner. If we are on the 
coast at the summit of a hill, and a vessel appears on the 
horizon (Fig. 66), we see only the tops of the masts and 
the highest sails ; the lower sails and the hull are invisible. 
As the vessel approaches, its lower part comes into view 
above the horizon, and soon it appears entire. 

In the same manner the sailors from the ship see the 
different parts of objects on the land appear successively, 
beginning with the highest. The reason of this will be 
evident from Fig. 67, where the course of a vessel, seen in 
profile, is figured on the convex surface of the sea. 

As the curvature of the ocean is the same in every direc- 
tion, it follows that the surface of the ocean is spherical. 




Fig. 6 7 . 

The same is true of the surface of the land, allowance being 
made for the various inequalities of the surface. From 
these and various other indications, we conclude that the 
earth is a spliere. 

56. Size of the Earth. — The size of the earth is ascer- 
tained by measuring the length of a degree of a meridian, 
and multiplying this by three hundred and sixty. This gives 
the circumference of the earth as about twenty-five thousand 
miles, and its diameter as about eight thousand miles. We 
know that the two stations between which we measure are 
one degree apart when the elevation of the pole at one 
station is one degree greater than at the other. 

57. The Earth Flattened at the Poles. — Degrees on the 
meridian have been measured in various parts of the earth, 
and it has been found that they invariably increase in length 



ASTRONOMY. 



56 

as we proceed from the equator towards the pole : hence 
the earth must curve less and less rapidly as we approach the 
poles ; for the less the curvature of a circle, the larger the 

degrees on it. 

58 The Earth in Space. — In Fig. 68 we have a view 
of the earth suspended in space. The side of the earth 







Fig. 68. 

turned towards the sun is illumined, and the other side is in 
darkness. As the planet rotates on its axis, successive por- 
tions of it will be turned towards the sun. As viewed from 
a point in space between it and the sun. it will present 
light and dark portions, which will assume different forms 
according to the portion which is illumined. These differ- 
ent appearances are shown in Fig. 69. 



ASTRONOMY, 



57 



Day axd Night. 
59. Day and Night. — The succession of day and night 




Fig. 69. 

is due to the rotation of the earth on its axis, by which a 
place on the surface of the earth is carried alternately into 
the sunshine and out of it. As the sun moves around the 



58 



ASTRONOMY. 



heavens on the ecliptic, it will be on the celestial equator 
when at the equinoxes, and 2$\° north of the equator when at 
the summer solstice, and 23^° south of the equator when 

at the winter solstice. 

60. Day and Night 
when the Sun is at the 
Equinoxes. — When the 
sun is at either equinox, 
the diurnal circle de- 
scribed by the sun will 
coincide with the celes- 
tial equator ; and there- 
fore half of this diurnal 
circle will be above the 
horizon at every point on 
Fi s- 70. the surface of the globe. 

At these times day and night will be equal in every part of 
the ear til. 




The equality of days and nights when the sun is on the 
celestial equator is also 
evident from the following 
considerations : one-half of 
the earth is in sunshine all 
of the time ; when the sun 
is on the celestial equator, 
it is directly over the equa- 
tor of the earth, and the 
illumination extends from 
pole to pole, as is evident 
from Figs. 70 and 71, in 
the former of which the 
sun is represented as on 
the eastern horizon at a 
place along the central line * g ' v " 

of the figure, and in the latter as on the meridian along 
the same line. In each diagram it is seen that the illumination 




ASTRONOMY. 



59 




extends from pole to pole : hence, as the earth rotates on its 
axis, every place on the surface will be in the sunshine and 
out of it just half of the time. 

61. Day and Night when the Sun is at the Summer 
Solstice. — When the sun is 
at the summer solstice, it 
will be 2$\° north of the 
celestial equator. The diur- 
nal circle described by the 
sun will then be 23^° north 
of the celestial equator ; and 
more than half of this diur- 
nal circle will be above the 
horizon at all places north 
of the equator, and less 
than half of it at places Fi s- 7 2 - 

south of the equator : hence the days will be longer than the 
nights at places north of the equator, and shorter than the 

nights at places south of 
the equator. At places 
within 23^° of the north 
pole, the entire diurnal 
circle described by the 
sun will be above the 
horizon, so that the sun 
will not set. At places 
within 23^° of the south 
pole of the earth, the en- 
tire diurnal circle will be 
below the horizon, so that 
Fig. 73. the sun will not rise. 

The illumination of the earth at this time is shown in 
Figs. 72 and 73. In Fig. 72 the sun is represented as on the 
western horizon along the middle line of the figure, and in 
Fig. 73 as on the meridian. It is seen at once that the illu- 




6 ASTRONOMY. 

mination extends 23^-° beyond the north pole, and falls 23^° 
short of the south pole. As the earth rotates on its axis, 
places near the north pole will be in the sunshine all the time, 
while places near the south pole will be out of the sunshine 
all the time. All places north of the equator will be in the 
sunshine longer than they are out of it, while all places south 
of the equator will be out of the sunshine longer than they 
are in it. 

62. Day and Night when the Sun is at the Winter Sol- 
stice. — When the sun is at the winter solstice, it is 23^° 
south of the celestial equator. The diurnal circle described 
by the sun is then 23^° south of the celestial equator. More 
than half of this diurnal circle will therefore be above the 
horizon at all places south of the equator, and less than 
half of it at all places north of the equator : hence the days 
will be longer than the nights south of the equator, and 
shorter than the nights at places north of the equator. At 
places within 23^-° of the south pole, the diurnal circle de- 
scribed by the sun will be entirely above the horizon, and 
the sun will therefore not set. At places within 23^° of the 
north pole, the diurnal circle described by the sun will be 
wholly below the horizon, and therefore the sun will not rise. 

The illumination of the earth at this time is shown in 
Figs. 74 and y$, and is seen to be the reverse of that shown 
in Figs. 72 and 73. 

63. Variation in the Length of Bay and Night. — As 
long as the sun is north of the equinoctial, the nights will 
be longer than the days south of the equator, and shorter 
than the days north of the equator. It is just the reverse 
when the sun is south of the equator. 

The farther the sun is from the equator, the greater is the 
inequality of the days and nights. 

The farther the place is from the equator, the greater the 
inequality of its days and nights. 

When the distance of a place from the north pole is less 



ASTRONOMY. 



61 




than the distance of the sun north of the equinoctial, it 
will have continuous day without night, since the whole of 
the sun's diurnal circle will be above the horizon. A place 
within the same distance of 
the south pole will have 
continuous night. 

When the distance of a 
place from the north pole is 
less than the distance of the 
.sun south of the equinoc- 
tial, it will have continuous 
night, since the whole of 
the sun's diurnal circle will 
then be below the horizon. 
A place within the same Flg- 74 ' 

distance of the south pole will then have continuous day. 

At the equator the days and nights arc always equal; 
since, no matter where the sun is in the heavens, half of 
all the diurnal circles described by it will be above the 

horizon, and half of them 
below it. 

64. The Zones. — It 
will be seen, from what 
has been stated above, 
that the sun will at some 
time during the year be 
directly overhead at every 
place within 23^° of the 
equator on either side. 
This belt of the earth is 
called the torrid zone. 
F{ s- 75- The torrid zone is bound- 

ed by circles called the tropics ; that of Cancer on the 
north, and that of Capricorn on the south. 

It will also be seen, that, at every place within 23^° ot 




62 ASTRONOMY. 

either pole, there will be, some time during the year,, a 
day during which the sun will not rise, or on which it will 
not set. These two belts of the earth's surface are called 
the frigid zones. These zones are bounded by the arctic 
circles. The nearer a place is to the poles, the greater the 
number of days on which the sun does not rise or set. 

Between the frigid zones and the torrid zones, there are 
two belts on the earth which are called the temperate zones. 
The sun is never overhead at any place in these two zones, 
but it rises and sets every day at every place within their 
limits. 

65. The Width of the Zones. — The distance the frigid 
zones extend from the poles, and the torrid zones from the 
equator, is exactly equal to the obliquity of the ecliptic, or the 
deviation of the axis of the earth from the perpendicular to 
the plane of its orbit. Were this deviation forty-five degrees, 
the obliquity of the ecliptic would be forty-five degrees, the 
torrid zone would extend forty-five degrees from the equator, 
and the frigid zones forty-five degrees from the poles. In 
this case there would be no temperate zones. Were this 
deviation fifty degrees, the torrid and frigid zones would 
overlap ten degrees, and there would be two belts of ten 
degrees on the earth, which would experience alternately 
during the year a torrid and a frigid climate. 

Were the axis of the earth perpendicular to the plane 
of the earth's orbit, there would be no zones on the earth, 
and no variation in the length of da}" and night. 

66. Twilight — Were it not for the atmosphere, the 
darkness of midnight would begin the moment the sun 
sank below the horizon, and would continue till he rose 
again above the horizon in the east, when the darkness of 
the night would be suddenly succeeded by the full light 
of day. The gradual transition from the light of day to 
the darkness of the night, and from the darkness of the 
night to the light of day, is called twilight, and is due to 



ASTRONOMY. 63 

the diffusion of light from the upper layers of the atmos- 
phere after the sun has ceased to shine on the lower layers 
at night, or before it has begun to shine on them in the 
morning. 

Let A BCD (Fig. 76) represent a portion of the earth, 
A a point on its surface where the sun S is setting ; and let 
SAN be a ray of light just grazing the earth at A, and 
leaving the atmosphere at the point H. The point A is 
illuminated by the whole reflective atmosphere HGFE. 
The point B, to which the sun has set, receives no direct 




Fig. 76. 



solar light, nor any reflected from that part of the atmos- 
phere which is below A L H ; but it receives a twilight from 
the portion HLF, which lies above the visible horizon B F. 
The point C receives a twilight only from the small portion 
of the atmosphere HMG ; while at D the twilight has 
ceased altogether. 

67. Duration of Twilight. — The astronomical limit of 
twilight is generally understood to be the instant when stars 
of the sixth magnitude begin to be visible in the zenith at 
evening, or disappear in the morning. 

Twilight is usually reckoned to last until the sun's depres- 
sion below the horizon amounts to eighteen degrees : this, how- 
ever, varies: in the tropics a depression of sixteen or seventeen 
degrees being sufficient to put an end to the phenomenon, 
while in England a depression of seventeen to twenty-one 
degrees is required. The duration of twilight differs in differ- 



64 ASTRONOMY. 

ent latitudes; it varies also in the same latitude at different 
seasons of the year, and depends, in some measure, on the 
meteorological condition of the atmosphere. When the sky 
is of a pale color, indicating the presence of an unusual 
amount of condensed vapor, twilight is of longer duration. 
This happens habitually in the polar regions. On the contrary, 
within the tropics, where the air is pure and dry. twilight some- 
times lasts only fifteen minutes. Strictly speaking, in the lati- 
tude of Greenwich there is no true night from May 22 to 
July 21, but constant twilight from sunset' to sunrise. Twilight 
reaches its minimum three weeks before the vernal equinox, 
and three weeks after the autumnal equinox, when its duration 
is an hour and fifty minutes. At midwinter it is longer by 
about seventeen minutes ; but the augmentation is frequently 
not perceptible, owing to the greater prevalence of clouds and 
haze at that season of the year, which intercept the light, and 
hinder it from reaching the earth. The duration is least at 
the equator (an hour and twelve minutes), and increases as 
we approach the poles ; for at the former there are two twi- 
lights every twenty-four hours, but at the latter only two in a 
year, each lasting about fifty days. At the north pole the sun 
is below the horizon for six months, but from Jan. 29 to the 
vernal equinox, and from the autumnal equinox to Nov. 12, 
the sun is less than eighteen degrees below the horizon ; so 
that there is twilight during the whole of these intervals, and 
thus the length of the actual night is reduced to two months 
and a half. The length of the day in these regions is about 
six months, during the whole of which time the sun is con- 
stantly above the horizon. The general rule is, that to tJie 
inliabitants of an oblique sphere the twilight is longer in pro- 
portion as the place is nearer the elevated pole, and the sun is 
farther from the equator on the side of the elevated pole. 

The Seasons. 

68. The Seasons. — While the sun is north of the celes- 
tial equator, places north of the equator are receiving heat 
from the sun by day longer than they are losing it by radia- 
tion at night, while places south of the equator are losing 



ASTRONOMY. 



65 



heat by radiation at night longer than they are receiving it 
from the sun by day. When, therefore, the sun passes north 
of the equator, the temperature begins to rise at places 
north of the equator, and to fall at places south of it. The 
rise of temperature is most rapid north of the equator when 
the sun is at the summer solstice ; but, for some time after 
this, the earth continues to receive more heat by day than it 
loses by night, and therefore the temperature continues to 
rise. For this reason, the heat is more excessive after the 
sun passes the summer solstice than before it reaches it. 

69. The Duration of the Seasons. — Summer is counted 
as beginning in June, when the sun is at the summer sol- 
stice, and as continuing until the sun reaches the autumnal 
equinox, in September. Autumn then begins, and continues 
until the sun is at the winter solstice, in December. Winter 
follows, continuing until the sun comes to the vernal equinox, 
in March, when spring 
begins, and continues 
to the summer sol- 
stice. In popular 



j^ifi 



reckoning 
sons begin 



the sea- 

with the 

of June, 

Decem- 




first day 

September, 

ber, and March. 

The reason why 
winter is counted as 
occurring after the 
winter solstice is simi- 
lar to the reason why 
the summer is placed 
after the summer solstice. The earth north of the equator 
is losing heat most rapidly at the time of the winter solstice ; 
but for some time after this it loses more heat by night than 
it receives by day : hence for some time the temperature 



66 



ASTRONOMY. 



continues to fall, and the cold is more intense after the 
winter solstice than before it. 




Of course, when it is summer in the northern hemisphere, 
it is winter in the southern hemisphere, and the reverse. 



ASTRONOMY. 



67 




Fig. 77 shows the portion of the earth's orbit included in 

each season. It will be seen that the earth is at perihelion 

in the winter season for 

places north of the 

equator, and at aphelion 

in the summer season. 

This tends to mitigate 

somewhat the extreme 

temperatures of our 

winters and summers. 

70. Th e I II it m in a tio n 
of the Earth at the 
different Seasons. — 
Fig. 78 shows the earth 
as it would appear to 
an observer at the sun Fi ?- 79. 

during each of the four seasons ; that is to say, the por- 
tion of the earth that is receiving the sun's rays. Figs. 79. 
80. 81, and 82 are enlarged views of the earth, as seen 

from the sun at the time 
of the summer solstice, of 
the autumnal equinox, of 
the winter solstice, and 
of the vernal equinox. 

Fig. 83 is, so to speak, 
a side view of the earth, 
showing the limit of sun- 
shine on the earth when 
the sun is at the summer 
solstice ; and Fig. 84. 
showing the limit of sun- 
shine when the sun is at 
the autumnal equinox. 

71. Cause of the Change of Seasons. — Variety in the 
length of day and night, and diversity in the seasons, depend 




Fig. 80. 



68 



ASTRONOMY. 



upon the obliquity of the ecliptic. 




Fig. 81. 



Were there no obliquity 
of the ecliptic, there 
would be no inequality 
in the length of day 
and night, and but 
slight diversity of sea- 
sons. The greater the 
obliquity of the eclip- 
tic, the greater would 
be the variation in the 
length of the days and 
nights, and the more 
extreme the 
of the seasons. 



changes 



Tides. 
72. Tides. — The alternate rise and fall of the surface of 
the sea twice in the course of a lunar day, or of twenty-four 
hours and fifty-one minutes, is known as the tides. When 
the water is rising, it is said to be flood tide ; and when 
it is falling, ebb tide. 
When the water is at its 
greatest height, it is said 
to be high water; and 
when at its least height, 
low water. 




73. Cause of the Tides. 
— It has been known to 
seafaring nations from a 
remote antiquity that there 
is a singular connection 
between the ebb and flow 
of the tides and the diur- 
nal motion of the moon. Fig " 82 * 

This tidal movement in seeming obedience to the moon was 
a mystery until the study of the law of gravitation showed it 



ASTRONOMY. 



6 9 



to be due to the attraction of the moon on the waters of the 
ocean. The reason why there are two tides a day will appear 




Let M be the moon, E the earth, and EM the 
line joining their centres. Now, strictly speaking, the moon 
does not revolve around the earth any more than the earth 




Fig 



around the moon : but the centre of each body moves around 
the common centre of ^ravitv of the two bodies. The earth 



yO ASTRONOMY. 

being eighty times as heavy as the moon, this centre is situated 
within the former, about three-quarters of the way from its 
centre to its surface, at the point G. The body of the earth 
itself being solid, every part of it, in consequence of the 
moon's attraction, may be considered as describing a circle 
once in a month, with a radius equal to EG. The centrifugal 
force caused by this rotation is just balanced by the mean 
attraction of the moon upon the earth. If this attraction were 
the same on every part of the earth, there would be even- 
where an exact balance between it and the centrifugal force. 
But as we pass from E to D the attraction of the moon dimin- 
ishes, owing to the increased distance : hence at D the centri- 
fugal force predominates, and the water therefore tends to move 
away from the centre E. As we pass from E towards C, the 
attraction of the moon increases, and therefore exceeds the cen- 




-e 



Fig. 85. 

trifugal force : consequently at C there is a tendency to draw 
the water towards the moon, but still away from the centre E. 
At A and B the attraction of the moon increases the gravity 
of the water, owing to the convergence of the lines BM and 
AM, along which it acts: hence the action of the moon tends 
to make the waters rise at D and C, and to fall at A and B, 
causing two tides to each apparent diurnal revolution of the 
moon. 

74- The Lagging of the Tides. — If the waters everywhere 
yielded immediately to the attractive force of the moon, it would 
always be high water when the moon was on the meridian, low 
water when she was rising or setting, and high water again 
when she was on the meridian below the horizon. But, owing 
to the inertia of the water, some time is necessary for so slight 
a force to set it in motion : and, once in motion, it continues 
so after the force has ceased, and until it has acted some time 
in the opposite direction. Therefore, if the motion of the 



ASTRONOMY. 



71 



water were unimpeded, it would not be high water until some 
hours after the moon had passed the meridian. The free 
motion of the water is also impeded by the islands and conti- 
nents. These deflect the tidal wave from its course in such a 




way that it may, in some cases, be many hours, or even a 
whole day, behind its time. Sometimes two waves meet , each 
other, and raise a very high tide. In some places the tides 




Fig. 87. 



run up a long bay, where the motion of a large mass of water 
will cause an enormous tide to be raised. In the Bay of 
Fundy both of these causes are combined. A tidal wave com- 
ing up the Atlantic coast meets the ocean wave from the 
east, and, entering the bay with their combined force, they 



72 ASTRONOMY. 

raise the water at the head of it to the height of sixty or 
seventy feet. 

75. Spring- Tides a nd Neap- Tides. — The sun produces 
a tide as well as the moon; but the tide-producing force 
of the sun is only about four-tenths of that of the moon. 
At new and fun moon the two bodies unite their forces, 
the ebb and flow become greater than the average, and 
we have the spring-tides. When the moon is in her first 




Fig. 88. 

or third quarter, the two forces act against each other; 
the tide-producing force is the difference of the two ; the 
ebb and flow are less than the average ; and we have the 
neap-tides. 

Fig. 86 shows the tide that would be produced by the 
moon alone; Fig. 87, the tide produced by the combined 
action of the sun and moon ; and Fig. 88, by the sun and 
moon acting at right angles to each other. 

The tide is affected by the distance of the moon from 



ASTRONOMY. 



73 



the earth, being highest near the time when the moon is in 
perigee, and lowest near the time when she is in apogee. 
When the moon is in perigee, at or near the time of a new 
or full moon, unusually high tides occur. 

76. Diurnal Inequality of Tides. — The height of the tide 
at a given place is influenced by the declination of the moon. 
When the moon has no declination, the highest tides should 
occur along the equator, and the heights should diminish from 
thence toward the north and south ; but the two daily tides at 
any place should have the same height. When the moon has 
north declination, as shown in Fig. 89, the highest tides on 
the side of the earth next the moon will be at places having 
a corresponding 
north latitude, 

as at B, and on x ' M. 

the opposite 
side at those 
which have an 
equal south lati- 
tude. Of the 
two daily tides 
at any place, that 
which occurs 
when the moon 
is nearest the 

zenith should be the greatest : hence, when the moon's declina- 
tion is north, the height of the tide at a place in north latitude 
should be greater when the moon is above the horizon than 
when she is below it. At the same time, places south of the 
equator have the highest tides when the moon is below the 
horizon, and the least when she is above it. This is called 
the diurnal inequality, because its cycle is one day; but it 
varies greatly in amount at different places. 

77. Height of Tides, — At small islands in mid-ocean 
the tides never rise to a great height, sometimes even less 
than one foot ; and the average height of the tides for the 
islands of the Atlantic and Pacific Oceans is only three feet 



TlS^F? 


§t^ 


^ 


^JI^jV 


-*Mfv 


~&Jg3% 


Wm: 


jsrs^ 


^K^E—S 


1= 


S 


w^ 




^ 





Jim 



Fig. 



^4 ASTRONOMY. 

and a half. Upon approaching an extensive coast where 
the water is shallow, the height of the tide is increased ; so 
that, while in mid-ocean the average height does not exceed 
three feet and a half, the average in the neighborhood of 
continents is not less than four or five feet. 

The Day and Time. 

78. The Day. — By the term day we sometimes denote 
the period of sunshine as contrasted with that of the absence 
of sunshine, which we call night, and sometimes the period 
of the earth's rotation on its axis. It is with the latter 
signification that the term is used in this section. As the 
earth rotates on its axis, it carries the meridian of a place 
with it ; so that, during each complete rotation of the earth, 
the portion of the meridian which passes overhead from 
pole to pole sweeps past every star in the heavens from west 
to east. The interval between two successive passages of 
this portion, of the meridian across the same star is the 
exact period of the complete rotation of the earth. This 
period is called a sidereal day. The sidereal day may also 
be defined as the interval between two successive passages 
of the same star across the meridian ; the passage of the 
meridian across the star, and the passage or transit of the 
star across the meridian, being the same thing looked at 
from a different point of view. The interval between two 
successive passages of the meridian across the sun, or of the 
sun across the meridian, is called a solar day. 

79. Length of the Solar Day. — The solar day is a little 
longer than the sidereal day. This is owing to the sun's 
eastward motion among the stars. We have already seen 
that the sun's apparent position among the stars is continu- 
ally shifting towards the east at a rate which causes it to 
make a complete circuit of the heavens in a year, or three 
hundred and sixty-five days. This is at the rate of about 
one degree a day : hence, were the sun and a star on the 



ASTRONOMY. 



75 



meridian together to-day, when the meridian again came 
around to the star, the sun would 
appear about one degree to the 
eastward : hence the meridian must 
be carried about one degree far- 
ther in order to come up to the 
sun. The solar day must there- 
fore be about four minutes longer 
tnan the sidereal day. 

The fact that the earth must 
make more than a complete rota- 
tion is also evident from Figs. 90 
and 91. In Fig. 90, ba represents 
the plane of the meridian, and the 
small arrows indicate the direction 
the earth is rotating on its axis, 
and revolving in its orbit. When FI s- 9°- 

the earth is at 1, the sun is on the meridian at a. When 





the earth has moved to 2, it has made a complete rotation, 
as is shown by the fact that the plane of the meridian is 



7 6 



ASTRONOMY. 



parallel with its position at i ; but it is evident that the 
meridian has not yet come up with the sun. In Fig. 91, 
O A represents the plane of the meridian, and OS the 
direction of the sun. The small arrows indicate the direc- 
tion of the rotation and 
revolution of the earth. In 
passing from the first posi- 
tion to the second the 
earth makes a complete 
rotation, but the meridian 
is not brought up to the 
sun. 

80. Inequality in the 
Length of Solar Days. — 
The sidereal days are all 
of the same length ; but the 







Fig. 92. 



solar days differ somewhat in length. This difference is due 
to the fact that the sun's apparent position moves eastward, 
or a7vay from the meridian, at a variable rate. 

There are three reasons why this rate is variable : — 

(1) The sun's eastward motion is 
due to the revolution of the earth 
in its orbit. Now, the earth's orbital 
motion is not uniform, being fast- 
est when the earth is at perihelion, 
and slowest when the earth is at 
aphelion : hence, other things being 
equal, solar days will be longest 
when the earth is at perihelion, 
and shortest when the earth is at 
aphelion. 

(2) The sun's eastward motion 
is along the ecliptic. Now, from Figs. 92 and 93, it will be 
seen, that, when the sun is at one of the equinoxes, it will 
be moving away from the meridian obliquely; and, from Figs. 
94 and 95, that, when the sun is at one of the solstices, it will 




ASTRONOMY. 



77 



be moving away from the meridian perpendicularly : hence, 
other things being equal, the sun would move away from the 
meridian fastest, and the days be longest, when the sun is at 
the solstices j while it would move away from the meridian 
slowest, and the days be shortest, when the sun is at the equi- 
noxes. That a body moving 
along the ecliptic must be 
moving at a variable angle to 
the meridian becomes very 
evident on turning a celestial 
globe so as to bring each 
portion of the ecliptic under 
the meridian in turn. 

(3) The sun, moving along 
the ecliptic, always moves in 
a great circle, while the point 
of the meridian which is to 
overtake the sun moves in a 
diurnal circle, which is some- 
times a great circle and sometimes a small circle. When the 
sun is at the equinoxes, the point of the meridian which is to 
overtake it moves in a great circle. As the sun passes from 

the equinoxes to the solstices, 
the point of the meridian which 
is to overtake it moves on a 
smaller and smaller circle : hence, 
as we pass away from the celes- 

- tial equator, the points of the 

meridian move slower and slower. 
Therefore, other things being 
equal, the meridian will gain 
upon the sun most rapidly, and 
the days be shortest, when the 
sun is at the equinoxes j while it 
will gain on the sun least rapidly, and the days will be longest, 
when the sun is at the solstices. 




Fig. 95. 



The ordinary or civil day is the mean of all the solar 
days in a year. 



y8 ASTRONOMY. 

Si. Sun Time and Clock Time. — It is noon by the sun 
when the sun is on the meridian, and by the clock at the 
middle of the civil day. Now, as the civil days are all of 
the same length, while solar days are of variable length, it 
seldom happens that the middles of these two days coincide, 
or that sun time and clock time agree. The difference 
between sun time and clock time, or what is often called 
apparent solar time and mean solar time, is called the equa- 
tion of time. The sun is said to be slow when it crosses the 
meridian after noon by the clock, and fast when it crosses 
the meridian before noon by the clock. Sun time and clock 
time coincide four times a year ; during two intermediate 
seasons the clock time is ahead, and during two it is behind. 

The following are the dates of coincidence and of maxi- 
mum deviation, which vary but slightly from year to year: — 

February 10 . . . True sun fifteen minutes slow. 

April 15 True sun correct. 

May 14 True sun four minutes fast. 

June 14 True sun correct. 

July 25 True sun six minutes slow. 

August 31 ... . True sun correct. 

November 2 . . . True sun sixteen minutes fast. 

December 24 . . . True sun correct. 

One of the effects of the equation of time which is fre- 
quently misunderstood is, that the interval from sunrise until 
noon, as given in the almanacs, is not the same as that between 
noon and sunset. The forenoon could not be longer or shorter 
than the afternoon, if by •■ noon " we meant the passage of the 
sun across the meridian ; but the noon of our clocks being 
sometimes fifteen minutes before or after noon by the sun, the 
former may be half an hour nearer to sunrise than to sunset, 
or vice versa. 

The Year. 

82. The Year. — The rear is the time it takes the earth 
to revolve around the sun, or, what amounts to the same 
thing, the time it takes the sun to pass around the ecliptic. 



ASTRONOMY. 



79 



(i) The time it takes the sun to pass from a star around 
to the same star again is called a sidereal year. This is. 
of course, the exact time it takes the earth to make a com- 
plete revolution around the sun. 

(2) The time it takes the sun to pass around from the 
vernal equinox, or the first 
point of Aries, to the vernal 
equinox again, is called the 
tropical year. This is a little 
shorter than the sidereal year, 
owing to the precession of 
the equinoxes. This will be 
evident from Fig. 96. The 
circle represents the ecliptic, 
S the sun, and E the vernal 
equinox. The sun moves 
around the eliptic eastward, as indicated by the long arrow, 
while the equinox moves slowly westward, as indicated by 
the short arrow. The sun will therefore meet the equinox 
before it has quite completed the circuit of the heavens. 
The exact lengths of these respective years are : — 






DAYS. 


HOURS. 


MIX. 


SEC. 


Sidereal year 


• 365^5636=365 


6 


9 


9 


Tropical year 


• 365^4220=365 


5 


48 


46 



Since the recurrence of the seasons depends on the tropi- 
cal year, the latter is the one to be used in forming the 
calendar and for the purposes of civil life generally. Its 
true length is eleven minutes and fourteen seconds less than 
three hundred and sixty-five days and a fourth. 

It will be seen that the tropical year is about twenty min- 
utes shorter than the sidereal year. 

(3) The time it takes the earth to pass from its perihelion 
point around to the perihelion point again is called the anoma- 
listic year. This year is about four minutes longer than the 
sidereal year. This is owing to the fact that the major axis of 



8o 



ASTRONOMY. 



the earth's orbit is slowly moving around to the east at the 
rate of about ten seconds a year. This causes the perihelion 
point P (Fig. 97) to move eastward at that rate, as indicated 
by the short' arrow. The earth E is also moving eastward, as 
indicated by the long arrow. Hence the earth, on starting at 
the perihelion, has to make a little more than a complete circuit 
to reach the perihelion point again. 

83. The Calendar. — The solar year, or the interval between 
two successive passages of the same equinox by the sun. 

is 365 days. 5 hours. 48 min- 
utes. 46 seconds. If, then, 
we reckon only 365 days to 
a common or civil year, the 
sun will come to the equinox 
EOl+P JH| I 5 hours. 48 minutes. 46 sec- 

Bl jjj' I onds. or nearly a quarter of 

V\ / a day. later each year: so 

that, if the sun entered Aries 

on the 20th of March one 

year, he would enter it on 

Flg - 97> the 2 1 st four years after, on 

the 22d eight years after, and so on. Thus in a comparatively 

short time the spring months would come in the winter, and 

the summer months in the spring. 

Among different ancient nations different methods of com- 
puting the year were in use. Some reckoned it bv the revo- 
lution of the moon, some by that of the sun: but none, so 
far as we know, made proper allowances for deficiencies and 
excesses. Twelve moons fell short of the true vear. thirteen 
exceeded it: 365 days were not enough. 366 were too many. 
To prevent the confusion resulting from these errors. Julius 
Caesar reformed the calendar by making the year consist of 
365 days. 6 hours (which is hence called a Julian yean, and 
made every fourth year consist of 366 days. This method of 
reckoning is called Old Style. 

But as this made the year somewhat too long, and the error 
in 1582 amounted to ten days. Pope Gregory XIII.. in order 
to bring the vernal equinox back to the 21st or March again. 
ordered ten days to be struck out of that vear. calling the next 




ASTRONOMY. 8 1 

day after the 4th of October the 15th: and. to prevent similar 
confusion in the future, he decreed that three leap-years should 
be omitted in the course of every four hundred years. This 
way of reckoning time is called New Style. It was immedi- 
ately adopted by most of the European nations, but was not 
accepted by the English until the year 1752. The error then 
amounted to eleven days, which were taken from the month of 
September by calling the 3d of that month the 14th. The Old 
Style is still retained by Russia. 

According to the Gregorian calendar, every year whose man- 
ner is divisible by four is a leap-year, except, that, in the case 
of the years w/iose numbers are exact hundreds, those only are 
leap-years which arc divisible by four after cutting off the last 
two figures. Thus the years 1600, 2000. 2400. etc., are leap- 
years: 1700, 1800. 1900. 2100. 2200. etc., are not. The error 
will not amount to a day in over three thousand years. 

84. The Dominical Letter. — The dominical letter for any 
year is that which we often see placed against Sunday in the 
almanacs, and is always one of the first seven in the alphabet. 
Since a common year consists of 365 clays, if this number is 
divided by seven (the number of days in a week), there will be 
a remainder of one : hence a year commonly begins one day 
later in the week than the preceding one did. If a year of 
365 days begins on Sunday, the next will begin on Monday: 
if it begins on Thursday, the next will begin on Friday: and 
so on. If Sunday falls on the 1st of January, the first letter 
of the alphabet, or A. is the dominical letter. If Sunday falls 
on the 7th of January (as it will the next year, unless the first 
is leap-vean. the seventh letter. G. is the dominical letter. If 
Sunday falls on the 6th of January (as it will the third year, 
unless the first or second is leap-year), the sixth letter. F, will 
be the dominical letter. Thus, if there were no leap-years, the 
dominical letters would regularly follow a retrograde order. 
G, F, E, D. C. B. A. 

But leap-years have 366 days, which, divided by seven, 
leaves two remainder : hence the years following leap-years will 
begin two days later in the week than the leap-years did. To 
prevent the interruption which would hence occur in the order 
of the dominical letters, leap-years have two dominical letters, 



82 



ASTRONOMY. 



one indicating Sunday till the 29th of February, and the other 
for the rest of the year. 

By Table I. below, the dominical letter for any year (New 



Table I. 


Table II. 




Centuries. 

j 




ABC D E 


F 


G 


' 100 200 300 400 




; 5OO 600 7OO, 800J| 


12345 


6 


7 




1 900 IOOO I IOO I200 


Jan. 31. 


8 9 10 11 12 


13 


H 


Years less th 


I300 I4OO I5OO l600 

in 1700 1800 1900 2000 




15 16 17 18 19 


20 


21 


One Hundre 


2100 2200 230c 
'■*• 2500 2600 270c 


)|2400 

2800 


Oct. 31. 


22 23 24 25 26 
29 30 31 .. .. 


27 


28 

j • • 


290c 


> 3000 310c 


3200 




1 2 


3 


4 




3300 3400 350c 


3600 J,- ebi 2S-29. 

4000 i 


56789 




3700 3800 390c 


10 


11 




! C E G 


BA 


; March 31. 
Nov. 30. 


12 13 14 15 16 
19 20 21 22 23 
26 27 28 29 30 


17 

r 2 « 


18 
25 


I 29 


S7 ^ 


$S B D 


F 


G 


2 ^0 


58 \ 

59 * 


36 A 

l 7 G 


C 
B 


E 

D 


F 
E 


31 


* 


3 


31 








1 


4 


32 


60 \ 


IS FE 


AG 


CB 


Dc i April 30. 


2 3! 4 5:6 


7 


8! 


5 


S3 


61 I 


>9 D 


F 


A 


B 


9 10 


11 12 13 
18 19 20 


14 


15! 


6 


34 


62 c 


>o C 


E 


G A Tulv 31 


16 17 


21 


22) 


7 


35 


63 c 


>i B 


D 


F G 




23 24 


25 26 27 


28 


2 9 , 


8 


36 


64 c 


2 AG 


CB 


EDFE 




3° 3 1 


. " ' | 






9 


37 


6s c 


3 F 


A 


C 


D 




.... 
6 7 


1 2 3 
8 9 10 


4 


5 


10 


38 


66 c 


4 E 


G 


B 


C 




11 


12 


11 


39 


67 jq 


S D 


F 


A 


B 


Aug. 31. 


13 14 


15 16 17 


18 


19 


12 


40 


68 c 


6 CB 


ED 


GF 


AG 




20 21 


22 23 24 


25 


2b 


^3 


41 


69 9 

70 q 


7 A 

8 G 


C 
B 


E 
D 


F 




27 28 


29; JO 131 


1 


~2 




42 


E 




5 6 7 






15 


43 


71 q 


9! F 


A 


C 


D Sept. 30. 
CB 


3 4 


6 


9! 


ib 


44 


72 . 


.ED 


GF 


BA 


10 11 


12 13 14 


15 


16 


17 

18 


45 
46 


73 • 

74 • 


IT 

. B 


E 
D 


G 
F 


X Dec - 3i- 
G 


17 18 19 20 
24 25 26 27 

31 ..(.■ - 


21 

28 


22 
29 


23 
30 


19 
20 


47 
48 


75 • 

76 . 


A 


C 
BA 


E 

DC 


TT ! ■ i- 






' ' 


. GF; 


1 - j 

ED 


..123 


4 


5 


6 


21 

22 
23 


49 
50 
Si 


77 • 

78 . 

79 • 


E 

. D 
. C 


G 
F 
E 


B 

A 
G 




C May 31. 

B J 
A 


7 8 9 10 
14 15 16 17 
21 22 23 24 
28 29 30 31 


11 

18 

25 


12 

19 
26 


13 
20 

27 


24 


S2 


80 . 


. BADC 


FF 


GFi! — = — - 








2S 


S3 


81 T 


. G IT 


TT 


__ 1 
E 


4 5 6 

11 12 13 
18 19 20 
25 26 27 




1 

8 

15 


2 3 


26 

27 


54 

ss 


82 . 

83 • 


. F,A 

.EG 


c 

B 


D 

c 


June 30. 


7 


9 10 
16! 17 


28 


56 


84 . 


. DC 


FE 


AGI 


BA; 




21 22 
28129 


23,24 
30 .. 



Style) for four thousand years from the beginning of the Chris- 
tian Era may be found ; and it will be readily seen how the 



ASTRONOMY. 83 

Table could be extended indefinitely by continuing the centu- 
ries at the top in the same order. 

To find the dominical letter by this table, look for the hun- 
dreds of years at the top, and for the years below a hundred, 
at the left hand. 

Thus the letter for 1882 will be opposite the number 82, 
and in the column having 1800 at the top; that is, it will be A. 
In the same way, the letters for 1884, which is a leap-year, will 
be found to be FE. 

Having the dominical letter of any year, Table II shows 
what days of every month of the year will be Sundays. 

To find the Sundays of any month in the year by this table, 
look in the column, under the dominical letter, opposite the 
name of the month given at the left. 

From the Sundays the date of any other day of the week 
can be readily found. 

Thus, if we wish to know on what day of the week Christ- 
mas falls in 1889, we look opposite December, under the letter 
F (which we have found to be the dominical letter for the year), 
and find that the 22d of the month is a Sunday; the 25th. or 
Christmas, will then be Wednesday. 

In the same way we may find the day of the week corre- 
sponding to any date (New Style) in history. For instance, the 
17th of June, 1775, the day °f the fight at Bunker Hill, is found 
to have been a Saturday. 

These two tables then serve as a perpetual almanac. 

Weight of the Earth and Precession. 

85. The Weight of the Earth. — There are several methods 
of ascertaining the weight and mass of the earth. The sim- 
plest, and perhaps the most trustworthy method is to compare 
the pull of the earth upon a ball of lead with that of a known 
mass of lead upon it. The pull of a known mass of lead upon 
the ball may be measured by means of a torsion balance. One 
form of the balance employed for this purpose is shown in 
Figs. 98 and 99. Two small balls of lead, b and b. are fastened 
to the ends of a light rod e, which is suspended from the point 
F by means of the thread FE. Two large balls of lead, W 
and W) are placed on a turn-table, so that one of them shall 



8 4 



ASTRONOMY. 



be just in front of one of the small balls, and the other just 
behind the other small ball. The pull of the large balls turns 
the rod around a little so as to bring the small balls nearer the 
large ones. The small balls move towards the large ones till 




they are stopped by the torsion of the thread, which is then 
equal to the pull of the large balls. The deflection of the rod 
is carefully measured. The table is then turned into the posi- 
tion indicated by the dotted lines in Fig. 99, so as to reverse 
the position of the large balls with reference to the small ones. 




Fig. 99. 

The rod is now deflected in the opposite direction, and the 
amount of deflection is again carefully measured. The second 
measurement is made as a check upon the accuracy of the first. 
The force required to twist the thread as much as it was 



ASTRONOMY. 85 

twisted by the deflection of the rod is ascertained by measure- 
ment. This gives the pull of the two large balls upon the two 
small ones. We next calculate, what this pull would be were 
the balls as far apart as the small balls are from the centre of 
the earth. We can then form the following proportion: the 
pull of the large balls upon the small ones is to the pull of the 
earth upon the small ones as the mass of the large balls is to 
the mass of the earth, or as the weight of the large balls is 
to the weight of the earth. Of course, the pull of the earth 
upon the small balls is the weight of the small balls. In this 
way it has been ascertained that the mass of the earth is about 
5.6 times that of a globe of water of the same size. In other 
words, the mean density of the earth is about 5.6. 

The weight of the earth in pounds may be found by multi- 
plying the number of cubic feet in it by 6z\ (the weight, in 
pounds, of one cubic foot of water), and this product by 5.6. 

86. Cause of Precession. — We have seen that the earth is 



\8 



flattened at the poles : in other words, the earth has the form 
of a sphere, with a protuberant ring around its equator. This 
equatorial ring is inclined to the plane of the ecliptic at an angle 
of about 23I . In Fig. 100 this ring is represented as detached 
from the enclosed sphere. S represents the sun, and Sc the 
ecliptic. As the point A of the ring is nearer the sun than the 
point B is, the sun's pull upon A is greater than upon B : 
hence the sun tends to pull the ring over into the plane of the 
ecliptic : but the rotation of the earth tends to keep the ring in 
the same plane. The struggle between these two tendencies 
causes the earth, to which the ring is attached, to wabble like 
a spinning-top, whose rotation tends to keep it erect, while 
gravity tends to pull it over. The handle of the top has a 
gyratory motion, which causes it to describe a curve. The axis 
of the heavens corresponds to the handle of the top. 




86 ASTRONOMY. 

II. THE MOON. 

Distance, Size, and Motions. 

87. The Distance of the Moon. — The moon is the near- 
est of the heavenly bodies. Its distance from the centre of 
the earth is only about sixty times the radius of the earth, 
or, in round numbers, two hundred and forty thousand miles. 

The ordinary method of rinding the distance of one of the 
nearer heavenly bodies is first to ascertain its horizontal par- 
allax. This enables us to form a right-angled triangle, the 
lengths of whose sides are easily computed, and the length of 
whose hypothenuse is the distance of the body from the centre 
of the earth. 

Horizontal parallax has already been defined (32) as the dis- 
placement of a heavenly 
body when on' the horizon, 
caused by its being seen 
from the surface, instead of 
the centre, of the earth. 
This displacement is due 
to the fact that the body is seen in a different direction from 
the surface of the earth from that in which it would be seen 
from the centre. Horizontal parallax might be defined as the 
difference in the directions in which a body on the horizon 
would be seen from the surface and from the centre of the 
earth. Thus, in Fig. 101, C is the centre of the earth, A a 
point on the surface, and B a body on the horizon of A. A B 
is the direction in which the body would be seen from A, and 
CB the direction in which it would be seen from C. The dif- 
ference of these directions, or the angle ABC, is the parallax 
of the body. 

The triangle BA C is right-angled at A; the side A C is the 
radius of the earth, and the hypothenuse is the distance of the 
body from the centre of the earth. When the parallax ABC 
is known, the length of CB can easily by found bv trigono- 
metrical computation. 

We have seen (32) that the parallax of a heavenly body 




i 



ASTRONOMY. 87 

grows less and less as the body passes from the horizon 
towards the zenith. The parallax of a body and its altitude 
are, however, so related, that, when we know the parallax at 
any altitude, we can readily compute the horizontal parallax. 

The usual method of finding the parallax of one of the 
nearer heavenly bodies is first to find its parallax when on the 
meridian, as seen from two places on the earth which differ 
considerably in latitude : then to calculate what would be the 
parallax of the body as seen from one of these places and the 
centre of the earth ; and then finally to calculate what would 
be the parallax were the body on the horizon. 

Thus, we should ascertain the parallax of the body B (Fig. 
102) as seen from A and D. or the angle ABB. We should 
then calculate its parallax as seen from A and C. or the angle 
ABC. Finally we should calculate what its parallax would be 
were the body on the horizon, or the angle A B' C. 

The simplest method of 
finding the parallax of a 
body B (Fig. 102) as seen 
from the two points A and 
D is to compare its direc- 
tion at each point with that 
of the same fixed star near 
the body. The star is so 
distant, that it will be seen 
in the same direction from 
both points : hence, if the 
direction of the body differs from that of the star 2 as seen 
from one point, and 2 6' as seen from the other point, the two 
lines AB and DB must differ in direction by 6': in other 
words, the angle ABD would be 6'. 

The method just described is the usual method of finding 
the parallax of the moon. 

88. The Apparent Size of the Moon. — The apparent size 
of a body is the visual angle subtended by it ; that is, the 
angle formed by two lines drawn from the eye to two oppo- 
site points on the outline of the body. The apparent size 
of a body depends upon both its magnitude and its distance. 




Fig. 102. 



88 



ASTRONOMY. 



The apparent size, or angular diameter, of the moon is 
about thirty-one minutes. This is ascertained by means of 
the wire micrometer already described (19). The instru- 
ment is adjusted so that its longitudinal wire shall pass 
through the centre of the moon, and its transverse wires 
==1 sna ^ ^ e tan g ent t0 the limbs of the 

Hmoon on each side, at the point 
iSS where they are cut by the longitu- 

dinal wire. The micrometer screw 
is then turned till the wires are 
brought together. The number of 
turns of the screw needed to accom- 
plish this will indicate the arc be- 
tween the wires, or the angular 
diameter of the moon. 



Fig. 103. 



In order to be certain that the longitudinal wire shall pass 
through the centre of the moon, it is best to take the moon 
when its disc is in the form of a crescent, and to place the 




longitudinal wire against the points, or cusps, of the crescent, 
as shown in Fig. 103. 

89. The Real Size of the Moon. — The real diameter of 
the moon is a little over one-fourth of that of the earth, or 
a little more than two thousand miles. The comparative 
sizes of the earth and moon are shown in Fig. 104. 



ASTRONOMY. 



89 



The distance and apparent size of the moon being known, 
her real diameter is found by means of a triangle formed as 
shown in Fig. 105. C represents the centre of the moon, CB 
the distance of the moon from the earth, and CA the radius of 
the moon. B A C is a triangle, right-angled at A. The angle 
ABC is half the apparent 
diameter of the moon. 
With the angles A and B, 
and the side CB known, it 
is easy to find the length 
of A C by trigonometrical 
computation. Twice A C will be the diameter of the moon. 




Fig. 105. 



The volume of the moon is about one-fiftieth of that of 
the earth. 

90. Apparent Size of the Moon on the Horizon and in 
the Zenith. — The moon is nearly four thousand miles far- 
ther from the observer when she is on the horizon than 
when she is in the zenith. This is evident from Fig. 106. 
C is the centre of the earth, M the moon on the hori- 
zon, M' the moon in the 
zenith, and O the point of 
observation. O M is the 
distance of the moon when 
she is on the horizon, and 
O M' the distance of the 
moon from the observer 
when she is in the zenith. 
CM is equal to CM', and 




Fig. 106. 



OM is about the length of CM; but O M r is about four 
thousand miles shorter than C M' : hence O M f is about 
four thousand miles shorter than OM. 

Notwithstanding the moon is much nearer when at the 
zenith than at the horizon, it seems to us much larger at the 
horizon. 

This is a pure illusion, as we become convinced when we 
measure the disc with accurate instruments, so as to make the 



90 



ASTRONOMY, 






result independent of our ordinary way of judging. When the 
moon is near the horizon, it seems placed beyond all the objects 
on the surface of the earth in that direction, and therefore far- 
ther off than at the zenith, where no intervening objects enable 
us to judge of its distance. In any case, an object which keeps 
the same apparent magnitude seems to us, through the instinc- 
tive habits of the eye. the larger in proportion as we judge it 
to be more distant. 

91. The Apparent Size of the Moon increased by Irradia- 

fi otlt i n the case of the moon, the word apparent means much 

more than it does in the case of other celestial bodies. Indeed, 
its brightness causes our eyes to play us false. As is well 
knowm the crescent of the new moon seems part of a much 




Fig. 107. 

larger sphere than that which it has been said, time out of mind, 
to "hold in its arms." The bright portion of the moon as seen 
with our measuring instruments, as well as when seen with the 
naked eye, covers a larger space in the field of the telescope 
than it would if it were not so bright. This effect of irradia- 
tion, as it is called, must be allowed for in exact measurements 
of the diameter of the moon. 

92. Apparent Size of the Moon in Different Parts of 
her Orbit. — Owing to the eccentricity of the moon's orbit, 
her distance from the earth varies somewhat from time to 
time. This variation causes a corresponding variation in 
her apparent size, which is illustrated in Fig. 107. 

93. The Mass of the Moon. — The moon is considerablv 



ASTRONOMY. gi 

less dense than the earth, its mass being only about one- 
eightieth of that of the earth ; that is, while it would take 
only about fifty moons to make the bulk of the earth, it 
would take about eighty to make the mass of the earth. 

One method of rinding the mass of the moon is to compare 
her effect in producing the tides with that of the sun. We 
first calculate what would be the moon's effect in producing the 
tides, were she as far off as the sun. We then form the follow- 
ing proportion: as the sun's effect in producing the tides is to 
the moon's effect at the same distance, so is the mass of the 
sun to the mass of the moon. 

The method of finding the mass of the sun will be given 
farther on. 

94. The Orbital Motion of the Moon. — If we watch 
the moon from night to night, we see that she moves east- 
ward quite rapidly among the' stars. When the new moon 
is first visible, it appears near the horizon in the west, just 
after sunset. A week later the moon will be on the meridian 
at the same hour, and about a week later still on the eastern 
horizon. The moon completes the circuit of the heavens 
in a period of about thirty days, moving eastward at the 
rate of about twelve degrees a day. This eastward motion 
of the moon is due to the fact that she is revolving around 
the earth from west to east. 

95. The Aspects of the Moon. — As the moon revolves 
around the earth, she comes into different positions with 
reference to the earth and sun. These different positions of 
the moon are called the aspects of the moon. The four 
chief aspects of the moon are shown in Fig. 108. When 
the moon is at M, she appears in the opposite part of the 
heavens to the sun, and is said to be in opposition ; when at 
M' and at M"\ she appears ninety degrees away from the 
sun. and is said to be in quadrature ; when at M r \ she 
appears in the same part of the heavens as the sun, and is 
said to be in conjunction. 



9 2 



ASTRONOMY. 



96. The Sidereal and Synodical Periods of the Moon. — 
The sidereal period of the moon is the time it takes her to 
pass around from a star to that star again, or the time it 

takes her to make 
a complete revolution 
around the earth. 
This is a period 
of about twenty-seven 
days and a third. It 
is sometimes called 
the sidereal month. 

The sxnodical peri- 
od of the moon is the 
time that it takes the 
moon to pass from 
one aspect around to the same aspect again. This is a 
period of about twenty-nine days and a half, and it is some- 
times called the sxnodical month. 




Fig. 1 




Fig. 109, 

The reason why the synodical period is longer than the 
sidereal period will appear from Fig. 109. 5 represents the 
position of the sun, E that of the earth, and the small 



ASTRONOMY. 93 

circle the orbit of the moon around the earth. The arrow 
in the small circle represents the direction the moon is 
revolving around the earth, and the arrow in the arc between 
E and E' indicates the direction of the earth's motion in 
its orbit. When the moon is at M l9 she is in conjunction. 
As the moon revolves around the earth, the earth moves 
forward in its orbit. When the moon has come round to 
0f l9 so that m ?t m v is parallel with M s M l9 she will have made 
a complete or sidereal revolution around the earth ; but 
she will not be in conjunction again till she has come round 
to M 9 so as again to be between the earth and sun. That 




is to say, the moon must make more than a complete revo- 
lution in a synodical period. 

The greater length of the synodical period is also evident 
from Fig. no. T represents the earth, and Z the moon. The 
arrows indicate the direction in which each is moving. When 
the earth is at T, and the moon at Z, the latter is in conjunc- 
tion. When the earth has reached T', and the moon Z', the 
latter has made a sidereal revolution ; but she will not be 
in conjunction again till the earth has reached T", and the 
moon L". 

97. The Phases of the Moon. — When the new moon 
appears in the west, it has the form of a crescent, with its 



94 ASTRONOMY. 

convex side towards the sun, and its horns towards the east. 




Fig in. 

As the moon advances towards quadrature, the crescent 
grows thicker and thicker, till it becomes a half-circle at 



ASTRONOMY. 95 

first quarter. When it passes quadrature, it begins to become 
convex also on the side away from the sun, or gibbons in 
form. As it approaches opposition, it becomes more and 
more nearly circular, until at opposition it is a full circle. 
From full moon to last quarter it is again gibbous, and at 
last quarter a half-circle. From last quarter to new moon 
it is again crescent ; but the horns of the crescent are now 
turned towards the west. The successive phases of the 
moon are shown in Fig. in. 

98. Cause of the Phases of the Moon. — Take a globe, 
half of which is colored white and the other half black in 
such a way that the line which separates the white and black 
portions shall be a great circle which passes through the 
poles of the globe, and rotate the globe slowly, so as to 
bring the white half gradually into view. When the white 
part first comes into view, the* line of separation between 
it and the black part, which we may call the tei'minator, 
appears concave, and its projection on a plane perpendicular 
to the line of vision is a concave line. As more and more 
of the white portion comes into view, the projection of the 
terminator becomes less and less concave. When half of 
the white portion comes into view, the terminator is pro- 
jected as a straight line. When more than half of the white 
portion comes into view, the terminator begins to appear as 
a convex line, and this line becomes more and more convex 
till the whole of the white half comes into view, when the 
terminator becomes circular. 

The moon is of itself a dark, opaque globe ; but the half 
that is towards the sun is always bright, as shown in Fig. 112. 
This bright half of the moon corresponds to the white half 
of the globe in the preceding illustration. As the moon 
revolves around the earth, different portions of this illumined 
half are turned towards the earth. At new moon, when the 
moon is in conjunction, the bright half is turned entirely 
away from the earth,, and the disc of the moon is black and 



96 



ASTRONOMY. 






invisible. Between new moon and first quarter, less than 
half of the illumined side is turned towards the earth, and 
we see this illumined portion projected as a crescent. At 
first quarter, just half of the illumined side is turned towards 
the earth, and we see this half projected as a half-circle. 




Between first quarter and full, more than half of the illu- 
mined side is turned towards the earth, and we see it as 
gibbous. At full, the whole of the illumined side is turned 
towards us, and we see it as a full circle. From full to new 
moon again, the phases occur in the reverse order. 



ASTRONOMY. 



97 



99. The For??i of the Mo oil's Orbit. — The orbit of the 
moon around the earth is an ellipse of slight eccentricity. 
The form of this ellipse is shown in Fig. 113. C is the 
centre of the ellipse, and E the position of the earth at one 
of its foci. The eccentricity of the ellipse is only about 
one-eighteenth. It is impossible for the eye to distinguish 
such an ellipse from a circle. 

100. The Inclination of the Moon's Orbit. — The plane 




of the moon's orbit is inclined to the ecliptic by an angle 
of about five degrees. The two points where the moon's 
orbit cuts the ecliptic are called her nodes. The moon's 
nodes have a westward motion corresponding to that of the 
equinoxes, but much more rapid. They complete the cir- 
cuit of the ecliptic in about nineteen years. 

The moon's latitude ranges from 5 ° north to 5 south; 
and since, owing to the motion of her nodes, the moon is, 



9 8 



ASTRONOMY. 






during a period of nineteen years, 5 north and 5 south of 
every part of the ecliptic, her declination will range from 
23.I- -f- 5 = 28I north to 2^ + 5 = 28|° south. 

10 1. The Meridian Altitude of the Moon.— The meridian 
altitude of any body is its altitude when on the meridian. 
In our latitude, the meridian altitude of any point on the 
equinoctial is forty-nine degrees. The meridian altitude of 
the summer solstice is 49 4- 23-I = 72V 5 , and that of the 
winter solstice is.49 — 2 3'h° = 2 S¥- The greatest meridian 



7H° + 5 = 



77i°, 



and its least 



>i°- 



altitude of the moon is 
meridian altitude, 25!° — 5^ = 20f 

When the moon's meridian altitude is greater than the 
elevation of the equinoctial, it is said to run high, and when 
less, to run low. The full moon runs high when the sun is 
south of the equinoctial, and low when the sun is north of 
the equinoctial. This is because the full moon is always in 
the opposite part of the heavens to the sun. 

102. Wet and Dry Moon. — At the time of new moon, the 
cusps of the crescent sometimes lie in a line which is nearly 
perpendicular with the horizon, and sometimes in a line which 
is nearly parallel with the horizon. In the former case the 
moon is popularly described as a wet moon, and in the latter 

case as a dry moon. 






v"W Horizon 



The great circle 
which passes through 
the centre of the sun 
and moon will pass 
through the centre of 
the crescent, and be 
perpendicular to the 
line joining the cusps. 
Now the ecliptic makes 
the least angle with the horizon when the vernal equinox is 
on the eastern horizon and the autumnal equinox is on the 
western. In our latitude, as we have seen, this angle is 25^°: 
hence in our latitude, if the moon were at new on the ecliptic 




ASTRONOMY. 



99 




when the sun is at the autumnal equinox, as shown at Mr. 
(Fig. 114), the great circle passing through the centre of the 
sun and moon would be the ecliptic, and at New York would 
be inclined to the horizon at an angle of 25I . If the moon 
happened to be 5 south of the ecliptic at this time, as at 
Af 4 , the great circle pass- 
ing through the centre of 
the sun and moon would 
make an angle of only 
20-^° with the horizon. 
In either of these cases 
the line joining the cusps 
would be nearly perpen- 
dicular to the horizon. 

If the moon were at 
new on the ecliptic when 
the sun is near the vernal 
equinox, as shown at M l 
(Fig. 115), the great circle 
passing through the centres of the sun and moon wtfuld make 
an angle of 72^° with the horizon at New York: and were the 
moon 5 north of the ecliptic at that time, as shown at M 2 , this 
great circle would make an angle of yj\° with the horizon. In 
either of these cases, the line joining the cusps would be nearly 
parallel with the horizon. 

At different times, the line joining the cusps may have every 
possible inclination to the horizon between the extreme cases 
shown in Figs. 114 and 115. 

103. Daily Retardation of the Moon's Rising. — The 
moon rises, on the average, about fifty minutes later each 
clay. This is owing to her eastward motion. As the moon 
makes a complete revolution around the earth in about 
twenty-seven days, she moves eastward at the rate of about 
thirteen degrees a day, or about twelve degrees a day faster 
than the sun. Were the moon, therefore, on the horizon 
at any hour to-day, she would be some twelve degrees below 
the horizon at the same hour to-morrow. Now, as the hori- 



IOO 



ASTRONOMY. 



zon moves at the rate of one degree in four minutes, it 
would take it some fifty minutes to come up to the moon sc 
as to bring her upon the horizon. Hence the daily retarda- 
tion of the moon's rising is about fifty minutes ; but it 

varies considerably in differ- 
ent parts of her orbit. 



There are two reasons for 
this variation in the daily 
retardation : — 

(r) The moon moves at 
varying rate in her orbit j her 
speed being greatest at perigee, 
and least at apogee : hence, 
other things being equal, the 
retardation is Greatest when 




Fig. 116. 



the moon is at perigee, and least when she is at apogee. 

(2) The moon moves at a varying angle to the horizon. 
The moon moves nearly in the plane of the ecliptic, and of 
course she passes both -equinoxes every lunation. When she 
is near the autumnal equinox, her path makes the greatest 
angle with the eastern horizon, and when she is near the 
vernal equinox, the least angle : 
hence the moon moves away 
from the horizon fastest when 
she is near the autumnal equi- 
nox, and slowest when she is 
near the vernal equinox. This 
will be evident from Figs. 116 
and 117. In each figure, SN 
represents a portion of the 
eastern horizon, and Ec, E' c' , 
a portion of the ecliptic. A E, 



S— . 




Fig. 117. 



in Fig. 116, represents the autumnal equinox, and AE M the 
daily motion of the moon. VE, in Fig. 117, represents the 
vernal equinox, and VE M' the motion of the moon for one 
day. In the first case this motion would carry the moon away 
from the horizon the distance A M, and in the second case the 



ASTRONOMY. 



IOI 



distance A'M'. Now, it is evident that AM is greater than 
A ' M' : hence, other things being equal, the greatest retardation 
of the moon's rising will be when the moon is near the autum- 
nal equinox, and the least retardation when the moon is near 
the vernal equinox. 

The least retardation at New York is twenty-three minutes, 
and the greatest an hour and seventeen minutes. The great- 
est and least retardations vary somewhat from month to 
month ; since they depend not only upon the position of the 
moon in her orbit with reference to the equinoxes, but also 




118. 



upon the latitude of the moon, and upon her nearness to 
the earth. 

The direction of the moon's motion with reference to 
the ecliptic is shown in Fig. 118, which shows the moon's 
motion for one day in July, 1876. 

104. The Harvest Moon. — The long and short retarda- 
tions in the rising of the moon, though they occur every 
month, are not likely to attract attention unless they occur 
at the time of full moon. The long retardations for full 
moon occur when the moon is near the autumnal equinox 
at full. As the full moon is always opposite to the sun, the 



!02 ASTRONOMY. 



sun must in this case be near the vernal equinox : hence 
the long retardations for full moon occur in the spring, the 
greatest retardation being in March. 

The least retardations for full moon occur when the moon 
is near the vernal equinox at full : the sun must then be 
near the autumnal equinox. Hence the least retardations for 
full moon occur in the months of August, September, and 
October. The retardation is, of course, least for September ; 
and the full moon of this month rises night after night less 
than half an hour later than the previous night. The full 
moon of September is called the " Harvest Moon," and that 
of October the " Hunter's Moon." 

105. The Rotation of the Moon. — A careful examina- 
tion of the spots on the disc of the moon reveals the fact 
that she always presents the same side to the earth. In 
order to do this, she must rotate on her axis while making a 
revolution around the earth, or in about twenty-seven days. 

106. Lib rations of the Moon. — The moon appears to 
rock slowly to and fro, so as to allow us to see alternately a 
little farther around to the right and the left, or above and 
below, than we otherwise could. This apparent rocking of 
the moon is called lib ration. The moon has three libra- 
tions : — 

(1) Lib ration in Latitude. — This libration enables us 
to see alternately a little way around on the northern and 
southern limbs of the moon. 

This libration is due to the fact that the axis of the moon 
is not quite perpendicular to the plane of her orbit. The 
deviation from the perpendicular is six degrees and a half. As 
the axis of the moon, like that of the earth, maintains the same 
direction, the poles of the moon will be turned alternately six 
degrees and a half toward and from the earth. 

(2) Libration in Longitude. — This libration enables us 
to see alternately a little farther around on the eastern and 
western limbs of the moon. 



= 



ASTRONOMY. 



I03 



It is due to the fact that the moon's axial motion is uniform, 
while her orbital motion is not. At perigee her orbital motion 
will be in advance of her axial motion, while at apogee the 
axial motion will be in advance of the orbital. In Fig. 119, 
E represents the earth. M the moon, the large arrow the 
direction of the moon's motion in her orbit, and the small 
arrow the direction of her motion of rotation. When the 
moon is at M, the line A B, drawn perpendicular to EM, 
represents the circle which divides the visible from the invisible 
portion of the moon. While the moon is passing from M to 
M\ the moon performs less than a quarter of a rotation, so 
that A B is no longer perpendicular to EM' . An observer on 
the earth can now see >T 

somewhat beyond A on 
the western limb of 
the moon, and not quite 
up to B on the eastern 
limb. While the moon 
is passing from M' to "m/( 
M". her axial motion 
again overtakes her or- 
bital motion, so that the 
line A B again becomes 
perpendicular to the 
line joining the centre 
of the moon to the 
centre of the earth. Exactly the same side is now turned 
towards the earth as when the moon was at M. While the 
moon passes from M" to M" f . her axial motion gets in advance 
of her orbital motion, so that A B is again inclined to the line 
joining the centres of the earth and moon. A portion of the 
eastern limb of the moon beyond B is now brought into view 
to the earth, and a portion of the western limb at A is carried 
out of view. While the moon is passing from M'" to M, the 
orbital motion again overtakes the axial motion, and AB is 
again perpendicular to ME. 




Fig. 119. 



(3) Parallactic Libration. — While an observer at the 
centre of the earth would get the same view of the moon, 



104 



ASTRONOMY. 



whether she were on the eastern horizon, in the zenith, 
or" on the western horizon, an observer on the surface of 
the earth does not get exactly the same view in these 
three cases. When the moon is on the eastern horizon. 
an observer on the surface of the earth would see a little 
farther around on the western limb of the moon than when 
she is in the zenith, and not quite so far around on the east- 
ern limb. On the contrary, when the moon is on the 
western horizon, an observer on the surface of the earth 
sees a little farther around on the eastern limb of the moon 
than when she is in the zenith, and not quite so far around 
on her western limb. 

This will be evident from Fig. 120. E is the centre of 

the earth, and O a 
point on its surface. 
AB is a line drawn 
through the centre 
of the moon, per- 
pendicular to a line 
;oining the centres 
of the moon and 
the earth. This line 
marks off the part 
of the moon turned 
towards the centre 
lg * ~ 2 °' of the earth, and re- 

mains essentially the same during the day. CD is a line drawn 
through the centre of the moon perpendicular to a line joining 
the centre of the moon and the point of observation. This 
line marks off the part of the moon turned towards O. When 
the moon is in the zenith. CD coincides with AB: but. when 
the moon is on the horizon. CD is inclined to AB. When the 
moon is on the eastern horizon, an observer at sees a little 
beyond B. and not quite to A; and. when she is on the western 
horizon, he sees a little beyond A, and not quite to B. B is 
on the western limb of the moon, and A on her eastern limb. 
Since this libration is due to the point from which the moon 







ASTRONOMY. 



I05 



is viewed, it is called parallactic libration : and. since it occurs 
daily, it is called diurnal libration. 

107. Portion of the Lunar Surface brought into View by 
Libration. — The area brought into view by the first two libra- 
tions is between one-twelfth and one-thirteenth of the whole 
lunar surface, or nearly one-sixth of the hemisphere of the moon 
which is turned away from the earth when the moon is at her 
state of mean libration. Of course a precisely equal portion 




Fig. 121. 

of the hemisphere turned towards us during mean libration is 
carried out of view by the lunar librations. 

If we add to each of these areas a fringe about one degree 
wide, due to the diurnal libration. and which we may call the 
parallactic fringe, we shall find that the total area brought into 
view is almost exactly one-eleventh part of the whole surface 
of the moon. A similar area is carried out of view: so that 
the whole region thus swayed out of and into view amounts 
to two-elevenths of the moon's surface. This area is shown 
in Fig. 121. which is a side view of the moon. 



io6 



ASTRONOMY. 



108. 77/£ Moons Path through Space. — Were the earth 
stationary, the moon would describe an ellipse around it similar 
to that of Fig. 113; but, as the earth moves forward in her 




Fig. 122. 

orbit at the same time that the moon revolves around it, the 
moon is made to describe a sinuous path, as shown by the 
continuous line in Fig. 122. This feature of the moon's path is 




Fig. 123. 

greatly exaggerated in the upper portion of the diagram. The 
form of her path is given with a greater degree of accuracy in 
the lower part of the figure (the broken line represents the path 



ASTRONOMY. 



I07 



of the earth): but even here there is considerable exaggeration. 
The complete serpentine path of the moon around the sun is 
shown, greatly exaggerated, in Fig. 123, the broken line being 
the path of the earth. 

The path described by the moon through space is much the 
same as that described by a point on the circumference of a 
wheel which is rolled over another wheel. If we place a cir- 
cular disk against the wall, and carefully roll along its edge 
another circular disk (to which a piece of lead pencil has been 




Fig. 124. 

fastened so as to mark upon the wall), the curve described will 
somewhat resemble that described by the moon. This curve 
is called an epicycloid, and it will be seen that at every point 
it is concave towards the centre of the larger disk. In the 
same way the moon's orbit is at eveiy point concave towards 
the sun. 

The exaggeration of the sinuosity in Fig. 123 will be more 
evident when it is stated, that, on the scale of Fig. 124, the 



io8 



ASTRONOMY. 



whole of the serpentine curve would lie within the breadth of 
the fine circular line MM', 

109. The Lunar Day.— The lunar day is twenty-nine 
times and a half as long as the terrestrial day. Near the 
moon's equator the sun shines without intermission nearly 
fifteen of our days, and is absent for the same length of 
time. Consequently, the vicissitudes of temperature to 
which the surface is exposed must be very great. During 
the long lunar night the temperature of a body on the 
moon's surface would probably fall lower than is ever known 
on the earth, while during the day it must rise higher than 
anywhere on our planet. 




Fig. 125. 

It might seem, that, since the moon rotates on her axis in 
about twenty-seven days, the lunar day ought to be twenty- 
seven days long, instead of twenty-nine. There is, however, 
a solar, as well as a sidereal, day at the moon, as on the earth ; 
and the solar day at the moon is longer than the sidereal da)', 
for the same reason as on the earth. During the solar day the 
moon must make both a synodical rotation and a synodical 
revolution. This will be evident from Fig. 125, in which is 
shown the path of the moon during one complete lunation. 
E, E\ E", etc., are the successive positions of the earth; and 
1, 2, 3, 4, 5, the successive positions of the moon. The small 
arrows indicate the direction of the moon's rotation. The 
moon is full at 1 and 5. At 1, A, at the centre of the moon's 



ASTRONOMY. IO9 

disk, will have the sun, which lies in the direction A S, upon the 
meridian. Before A will again have the sun on the meridian, 
the moon must have made a synodical revolution ; and, as will 
be seen by the dotted lines, she must have made more than a 
complete rotation. The rotation which brings the point A into 
the same relation to the earth and sun is called a synodical 
rotation. 

It will also be evident from this diagram that the moon must 
make a synodical rotation during a synodical revolution, in 
order always to present the same side to the earth. 

no. The Earth as seen from the Moon. — To an ob- 
server on the moon, the earth would be an immense moon, 
going through the same phases that the moon does to us ; 
but, instead of rising and setting, it would only oscillate to 
and fro through a few degrees. On the other side of the 
moon it would never be seen, at all. The peculiarities of 
the moon's motions which cause the librations, and make a 
spot on the moon's disk seem to an observer on the earth 
to oscillate to and fro, would cause the earth as a whole to 
appear to a lunar observer to oscillate to and fro in the 
heavens in a similar manner. 

It is a well-known fact, that, at the time of new moon, the 
dark part of the moon's surface is partially illumined, so 
that it becomes visible to the naked eye. This must be due 
to the light reflected to the moon from the earth. Since at 
new moon the moon is between the earth and sun, it follows, 
that, when it is new moon at the earth, it must be full earth 
at the moon : hence, while the bright crescent is enjoying 
full sunlight, the dark part of its surface is enjoying the 
light of the full earth. Fig. 126 represents the full earth as 
seen from the moon. 

The Atmosphere of the Moon. 
in. The Moon has no Appreciable Atmosphere. — There 
are several reasons for believing that the moon has little or 
no atmosphere. 



HO ASTRONOMY. 

(i) Had the moon an atmosphere, it would be indicated 




at the time of a solar eclipse, when the moon passes over 
the disk of the sun. If the atmosphere were of any con- 



ASTRONOMY. 



Ill 




siderable density, it would absorb a part of the sun's rays, 
so as to produce a dusky border in front of the moon's disk, 
as shown in Fig. 127. In reality no such dusky border is 
ever seen ; but the limb of the moon appears sharp, and 
clearly defined, as in 
Fig. 128. 

If the atmosphere 
were not dense enough 
to produce this dusky 
border, its refraction 
would be sufficient 
to distort the deli- 
cate cusps of the 
sun's crescent in the 
manner shown at 
the top of Fig. i2# ; g 
but no such distortion is ever observed. The cusps always 
appear clear and sharp, as shown at the bottom of the figure : 
hence it would seem that there can be no atmosphere of 

appreciable density at the 
moon. 

(2) The absence of an 
atmosphere from the moon 
is also shown by the ab- 
sence of twilight and of 
diffused daylight. 

Upon the earth, twilight 
continues until the sun is 
eighteen degrees below the 
horizon ; that is, day and 
night are separated by a 
belt twelve hundred miles in breadth, in which the transition 
from light to darkness is gradual. We have seen (66) that 
this twilight results from the refraction and reflection of 
light by our atmosphere : and, if the moon had an atmos- 




Fig. 128. 



112 



ASTRONOMY. 



phere, we should notice a similar gradual transition fron 
the bright to the dark portions of her surface. Such, how- 
ever, is not the case. The boundary between the light and 
darkness, though irregular, is sharply defined. Close to this 
boundary the unillumined portion of the moon appears just 
as dark as at any distance from it. 

The shadows on the moon are also pitchy black, without 
a trace of diffused daylight. 

(3) The absence of an atmosphere is also proved by the 
absence of refraction when the moon passes between us anc 
the stars. Let AB (Fig. 129) represent the disk of the moon, 
and CD an atmosphere supposed to surround it. Let SA * 
represent a straight line from the earth, touching the moon at 
A, and let S be a star situated in the direction of this line. If 

the moon had no 

E ------ ^^^^r ^-~ j r^ atmosphere, this 

star would appear 
to touch the edge 
of the moon at 
A; but, if the 
moon had an at- 
mosphere, a star behind the edge of the moon, at S', woulc 
be visible at the earth: for the ray S' A would be bent by the 
atmosphere into the direction A E'. So, also, on the opposite 
side of the moon, a star might be seen at the earth, although 
really behind the edge of the moon : hence.- if the moon had an 
atmosphere, the time during which a star would be concealed 
by the moon would be less than if it had no atmosphere, and 
the amount of this effect must be proportional to the -density 
of the atmosphere. 

The moon, in her orbital course across the heavens, is con- 
tinually passing before, or occulting, some of the stars that sc 
thickly stud her apparent path ; and when we see a star thus 
pass behind the lunar disk on one side, and come out again on 
the other side, we are virtually observing the setting and rising 

J O o o 

of that star upon the moon. The moon's apparent diameter 
has been measured over and over again, and is known with 




Fis;. 



ASTRONOMY. 113 

oreat accuracy : the rate of her motion across the sky is also 
known with perfect accuracy : hence it is easy to calculate how 
long the moon will take to travel across a part of the sky 
exactly equal in length to her own diameter. Supposing, then, 
that we observe a star pass behind the moon, and out again, it 
is clear, that, if there is no atmosphere, the interval of time 
during which it remains occulted ought to be exactly equal to 
the computed time which the moon would take to pass over the 
star. If, however, from the existence of a lunar atmosphere, 
the star disappears too- late, and re-appears too soon, as we 
have seen it would, these two intervals will not agree : the com- 
puted time will be greater than the observed time, and the 
difference will represent the amount of refraction the star's 
light has sustained or suffered, and hence the extent of atmos- 
phere it has had to pass through. 

Comparisons of these two intervals of time have been 
repeatedly made, the most extensive being executed under the 
direction of the Astronomer Royal of England, several years 
ago, and based upon no less than two hundred and ninety- 
six occultation observations. In this determination the meas- 
ured or telescopic diameter of the moon was compared with 
the diameter deduced from the occupations ; and it was found 
that the telescopic diameter was greater than the occultation 
diameter by two seconds of angular measurement, or by about 
a thousandth part of the whole diameter of the moon. This 
discrepancy is probably due, in part at least, to irradiation (91), 
which augments the apparent size of the moon, as seen in the 
telescope as well as with the naked eye ; but, if the whole two 
seconds were caused by atmospheric refraction, this would 
imply a horizontal refraction of one second, which is only one 
two-thousandth of the earth's horizontal refraction. It is pos- 
sible that an atmosphere competent to produce this refraction 
would not make itself visible in any other way. 

But an atmosphere two thousand times rarer than our air 
can scarcely be regarded as an atmosphere at all. The con- 
tents of an air-pump receiver can seldom be rarefied to a 
greater extent than to about a thousandth of the density of air 
at the earth's surface ; and the lunar atmosphere, if it exists 
at all, is thus proved to be twice as attenuated as what we 
commonly call a vacuum. 



H4 



ASTRONOMY. 



The Surface of the Moon. 

112. Dusky Patches on the Disk of the Moon. — With 
the naked eye, large dusky patches are seen on the moon, 
in which popular fancy has detected a resemblance to 
human face. With a telescope of low power, these dark 
patches appear as smooth as water, and they were once 




Fig. 130. 

supposed to be seas. This theory was the origin of the 
name mare (Latin for sea), which is still applied to the 
larger of these plains ; but, if there were water on the sur- 
face of the moon, it could not fail to manifest its presence 
by its vapor, which would form an appreciable atmosphere. 
Moreover, with a high telescopic power, these plains present 



ASTRONOMY. 



115 



a more or less uneven surface ; and, as the elevations and 
depressions are found to be permanent, they cannot, of 
course, belong to the surface of water. 

The chief of these plains are shown in Fig. 130. They are 
Mare Crisitim, Mare Fcectinditatis, Mare Nectaris, Mare Tran- 
quillitatis, Mare Serenitatis, Mare Imbrium, Ma7-e Frigoris, 
and Oceanus Procellarum. All these plains can easily be rec- 
ognized on the surface of 
the full moon with the un- 
aided eve. 

113. The Terminator 

of the Moon. — The 
terminator of the moon 
is the line which sepa- 
rates the bright and dark 
portions of its disk. 
When viewed with a 
telescope of even mod- 
erate power, the termi- 
nator is seen to be very 
irregular and uneven. 
Many bright points are 
seen just outside of the 
terminator in the dark 
portion of the disk, while 
all along in the neigh- 
borhood of the termi- 
nator are bright patches 
and dense shadows. These appearances are shown in Figs. 
131 and 132, which represent the moon near the first and 
last quarters. They indicate that the surface of the moon 
is very rough and uneven. 

As it is always either sunrise or sunset along the termi- 
nator, the bright spots outside of it are clearly the tops of 
mountains, which catch the rays of the sun while their bases 




Fig. 131. 



u6 



ASTRONOMY. 



are in the shade. The bright patches in the neighborhood 
of the terminator are the sides of- hills and mountains which 
are receiving the full light of the sun, while the dense 
shadows near by are cast by these elevations. 

114. Height of the Lunar Mountains. — There are two 
methods of finding the height of lunar mountains : — 







Fig. 132. 

(1) We may measure the length of the shadows, and 
then calculate the height of the mountains that would cast 
such shadows with the sun at the required height above the 
horizon. 

The length of a shadow may be obtained by the following 
method : the longitudinal wire of the micrometer (19) is adjusted 
so as to pass through the shadow whose length is to be meas- 






ASTRONOMY. 



117 



ured, and the transverse wires are placed one at each end of 
the shadow, as shown in Fig. 133. The micrometer screw is 
then turned till the wires are brought together, so as to ascer- 
tain the length of the arc between them. We may then form 
the proportion : the number of seconds in the semi-diameter 
of the moon is to the number of seconds in the length of the 
shadow, as the length of the moon's radius in miles to the 
length of the shadow in miles. 




Fig. 133- 

The height of the sun above the horizon is ascertained 
by measuring the angular distance of the mountain from the 
terminator. 

(2) We may measure the distance of a bright point from 
the terminator, and then construct a right-angled triangle, 
as shown in Fig. 134. A solution of this triangle will enable 
us to ascertain the height of the mountain whose top is just 
catching the level rays of the sun. 

B is the centre of the moon, M the top of the mountain, 



n8 



ASTRONOMY. 



and SA M a ray of sunlight which just grazes the terminator 
at A, and then strikes the top of the mountain at M. The 
triangle BAAfis right-angled at A. B A is the radius of the 
moon, and A M is known by measurement ; BAf, the hypothe- 
nuse, may then be found by computation. B M is evidently 
equal to the radius of the moon plus the height of the moun- 
tain. 

By one or the other of these methods, the heights of 
the lunar mountains have been found with a great degree 
of accuracy. It is claimed that the heights of the lunar 
mountains are more accurately known than those of the 




Fig. 134. 

mountains on the earth. Compared with the size of the 
moon, lunar mountains attain a greater height than those 
on the earth. 

115. General Aspect of the Lunar Surface. — A cursory 
examination of the moon with a low power is sufficient to 
show the prevalence of crater-like inequalities and the gen- 
eral tendency to circular shape which is apparent in nearly 
all the surface markings; for even the large " seas " and 
the smaller patches of the same character repeat in their 
outlines the round form of the craters. It is along the 
terminator that we see these crater-like spots to the best 
advantage ; as it is there that the rising or setting sun casts 



ASTRONOMY. 



II 9 



long shadows over the lunar landscape, and brings eleva- 
tions into bold relief. They vary greatly in size ; some being 
so large as to bear a sensible proportion to the moon's 
diameter, while the smallest are so minute as to need the 
most powerful telescopes and the finest conditions of atmos- 
phere to perceive them. 
The prevalence of ring-shaped mountains and plains will 




Fig. 135- 

be evident from Fig. 135. which is from a photograph of a 
model of the moon constructed by Nasmyth. 

This same feature is nearly as marked in Figs. 131 and 
132, which are copies of Rutherford's photographs of the 
moon. 

116. Lunar Craters. — The smaller saucer-shaped forma- 
tions on the surface of the moon are called craters. They 



120 



ASTRONOMY. 



are of all sizes, from a mile to a hundred and fifty miles in 
diameter ; and they are supposed to be of volcanic origin. 
A high telescopic power shows that these craters vary re- 
markably, not only in size, but also in structure and arrange- 
ment. Some are considerably elevated above the surrounding 
surface, others are basins hollowed out of that surface, and 
with low surrounding ramparts ; some are like walled plains, 
while the majority have their lowest depression considerably 
below the surrounding surface ; some are isolated upon the 
plains, others are thickly crowded together, overlapping and 
intruding upon each other; some have elevated peaks or 
cones in their centres, and some are without these central 
cones, while others, again, contain several minute craters 
instead ; some have their ramparts whole and perfect, others 
have them broken or deformed, and many have them 
divided into terraces, especially on their inner sides. 

A typical lunar crater is shown in Fig. 136. 

It is not generally believed that any active volcanoes exist 
on the moon at the present time, though some observers 
have thought they discerned indications of such volcanoes. 

117. Copernicus. — This is one of the grandest of lunar 
craters (Fig. 137). Although its diameter (forty-six miles) 
is exceeded by others, yet, taken as a whole, it forms one of 
the most impressive and interesting objects of its class. 
Its situation, near the centre of the lunar disk, renders all 
its wonderful details conspicuous, as well as those of objects 
immediately surrounding it. Its vast rampart rises to up- 
wards of twelve thousand feet above the level of the plateau, 
nearly in the centre of which stands a magnificent group of 
cones, three of which attain a height of more than twenty- 
four hundred feet. 

Many ridges, or spurs, may be observed leading away from 
the outer banks of the great rampart. Around the crater, 
extending to a distance of more than a hundred miles on 
every side, there is a complex network of bright streaks, 



ASTRONOMY. 121 

which diverge in all directions. These streaks do not 




appear in the figure, nor are they seen upon the moon, 



122 



ASTRONOMY, 



except at and near the full phase. They show conspicu- 
ously, however, by their united lustre on the full moon. 

This crater is seen just to the south-west of the large 
dusky plain in the upper part of Fig. 132. This plain is 
Mare Imbrium, and the mountain-chain seen a little to the 
right of Copernicus is named the Apennines, Copernicus 




Fig. 137. 



is also seen in Fig. 135, a little to the left of the same 
range. 

Under circumstances specially favorable, myriads of com- 
paratively minute but perfectly formed craters may be ob- 
served for more than seventy miles on all sides around 
Copernicus. The district on the south-east side is specially 
rich in these thickly scattered craters, which we have reason 
to suppose stand over or upon the bright streaks. 



*L 






ASTRONOMY. 



I23 



118. Dark Chasms. — Dark cracks, or chasms, have been 
observed on various parts of the moon's surface. They 
sometimes occur singly, and sometimes in groups. They are 
often seen to radiate from some central cone, and they 
appear to be of volcanic origin. They have been called 
canals and rills. 




Fig. 138. 

One of the most remarkable groups of these chasms is 
that to the west of the crater named Triesneker. The 
crater and the chasms are shown in Fig. 138. Several of 
these great cracks obviously diverge from a small crater near 
the west bank of the great one, and they subdivide as they 
extend from the apparent point of divergence, while they 
are crossed by others. These cracks, or chasms, are nearly 



124 



ASTRONOMY. 



a mile broad at the widest part, and, after extending full a 
hundred miles, taper away till they become invisible. 

119. Mountain-Ranges. — There are comparatively few 
mountain-ranges on the moon. The three most conspicuous 




are those which partially enclose Mare Imbrium ; namely, 
the Apennines on the south, and the Caucasus and the 
Alps on the east and north-east. The Apennines are the 
most extended of these, having a length of about four hun- 



ASTRONOMY. 



125 



dred and fifty miles. They rise gradually, from a compara- 
tively level surface towards the south-west, in the form of 
innumerable small elevations, which increase in number and 




height towards the north-east, where they culminate in a 
range of peaks whose altitude and rugged aspect must form 
one of the most terribly grand and romantic scenes which 



126 



ASTRONOMY, 



imagination can conceive. The north-east face of the range 
terminates abruptly in an almost vertical precipice ; while 
over the plain beneath, intensely black spire-like shadows 
are cast, some of which at sunrise extend full ninety miles, 
till they lose themselves in the general shading due to the 
curvature of the lunar surface. Many of the peaks rise to 




heights of from eighteen thousand to twenty thousand feet 
above the plain at their north-east base (Fig. 139). 

Fig. 140 represents an ideal lunar landscape near the base 
of such a lunar range. Owing to the absence of an atmos- 
phere, the stars will be visible in full daylight. 

120. The Valley of the Alps. — The range of the Alps 
is shown in Fig. 141. The great crater at the north end of 
this range is named Plato. It is seventy miles in diameter. 



ASTRONOMY, 



127 



The most remarkable feature of the Alps is the valley 
near the centre of the range. It is more than seventy-five 
miles long, and about six miles wide at the broadest part. 
When examined under favorable circumstances, with a high 
magnifying power, it is seen to be a vast flat-bottomed 
valley, bordered by gigantic mountains, some of which 
attain heights of ten thousand feet or more. 

121. Isolated Peaks. — There are comparatively few 
isolated peaks to be found on the surface of the moon. 
One of the most remarkable of these is that known as Pico, 




Fig. 142. 

and shown in Fig. 142. Its height exceeds eight thousand 
feet, and it is about three times as long at the base as it is 
broad. The summit is cleft into three peaks, as is shown 
by the three-peaked shadow it casts on the plain. 

122. Bright Rays. — About the time of full moon, with 
a telescope of moderate power, a number of bright lines 
may be seen radiating from several of the lunar craters, 
extending often to the distance of hundreds of miles. 
These streaks do not arise from any perceptible difference 
of level of 4he surface, they have no very definite outline, 



128 



ASTRONOMY. 



and they do not present any sloping sides to catch more 
sunlight, and thus shine brighter, than the general surface. 
Indeed, one great peculiarity of them is, that they come out 
most forcibly when the sun is shining perpendicularly upon 
them : hence they are best seen when the moon is at full, 




Fig- 143. 



and they are not visible at all at those regions upon which 
the sun is rising or setting. They are not diverted by eleva- 
tions in their path, but traverse in their course craters, 
mountains, and plains alike, giving a slight additional bright- 
ness to all objects over which they pass, but producing no 



ASTRONOMY. 1 29 

other effect upon them. " They look as if, after the whole 
surface of the moon had assumed its final configuration, a 
vast brush charged with a whitish pigment had been drawn 
over the globe in straight lines, radiating from a central 
point, leaving its trail upon every thing it touched, but 
obscuring nothing." 

The three most conspicuous craters from which these 
lines radiate are Tycho, Copernicus, and Keplei\ Tycho is 
seen at the bottom of Figs. 143 and 130. Kepler is a little 
to the left of Copernicus in the same figures. 

It has been thought that these bright streaks are chasms 
which have been filled with molten lava, which, on cooling, 
would afford a smooth reflecting surface on the top. 

123. Tycho. — This crater is fifty-four miles in diameter, 
and about sixteen thousand feet deep, from the highest ridge 
of the rampart to the surface, of the plateau, whence rises a 
central cone five thousand feet high. It is one of the most 
conspicuous of all the lunar craters ; not so much on 
account of its dimensions as from its being the centre from 
whence diverge those remarkable bright streaks, many of 
which may be traced over a thousand miles of the moon's 
surface (Fig. 143). Tycho appears to be an instance of a 
vast disruptive action which rent the solid crust of the moon 
into radiating fissures, which were subsequently filled with 
molten matter, whose superior luminosity marks the course 
of the cracks in all directions from the crater as their 
common centre. So numerous are these bright streaks 
when examined by the aid of the telescope, and they give 
to this region of the moon's surface such increased lumi- 
nosity, that, when viewed as a whole, the locality can be 
distinctly seen at full moon by the unassisted eye, as a 
bright patch of light on the southern portion of the disk. 



130 



ASTRONOMY, 



III. INFERIOR AND SUPERIOR PLANETS. 



124. 




Inferior Planets. 

The Inferior Planets.— The inferior planets are 

those which lie be- 
tween the earth and 
the sun. and whose 
orbits are included 
by that of the earth. 
They are Mercury 
and Venus. 

125. Aspects of an 
Inferior Planet. — 
The four chief aspects 
of an inferior planet 
as seen from the 
earth are shown in 
Fi §- x 44- Fig. 144, in which 5 

represents the sun, P the planet, and E the earth. 
When the planet is 

between the earth and 

the sun, as at P, it 

is said to be in infe- 
rior conjunction. 
When it is in the 

same direction as the 

sun, but beyond it, 

as at P h \ it is said to 

be in superior con- 

j unction. 

When the planet is 

at such a point in 

its orbit that a line Fi §- *45- 

drawn from the earth to it would be tangent to the orbit, 




as at P' and P" 



it is said to be at its greatest elongation. 



ASTRONOMY. 



'31 



1 26. Apparent Motion of an Inferior Pla7iet. — When the 
planet is at P, if it could be seen at all, it would appear 
in the heavens at A. As it moves from P to P\ it will 
appear to move in the heavens from A to B. Then, as it 
moves from P r to P\ it will appear to move back again 
from B to A. While it moves from P" to P"\ it will appear 
to move from A to C ; and, while moving from P" r to P, 




it will appear to move back again from C to A. Thus the 
planet will appear to oscillate to and fro across the sun from 
B to C, never getting farther from the sun than B on the 
west, or C on the east : hence, when at these points, it is 
said to be at its greatest western and eastern elongations. 
This oscillating motion of an inferior planet across the sun, 
combined with the sun's motion anions: the stars, causes the 



13- 



ASTRONOMY. 



planet to describe a path among the stars similar to that 
shown in Fig. 145. 

127. Phases of an Inferior Planet. — An inferior planet, 
when viewed with a telescope, is found to present a succes- 
sion of phases similar to those of the moon. The reason 
of this is evident from Fig. 146. As an inferior planet 
passes around the sun. it presents sometimes more and 
sometimes less of its bright hemisphere to the earth. When 
the earth is at T. and Venus at superior conjunction, the 
planet turns the whole of its bright hemisphere towards the 
earth, and appears full ; it then becomes gibbous, half and 
crescent. When it comes into inferior conjunction, it turns 

its dark hemisphere towards 



the earth : it then becomes 
crescent, half gibbous, and 
full again. 

128. The Sidereal and 
Sy nodical Periods of an Infe- 
rior Planet. — The time it 
takes a planet to make a 
complete revolution around 
the sun is called the side- 
real period of the planet; 
and the time it takes it to pass from one aspect around to 
the same aspect again, its synodical period. 

The synodical period of an inferior planet is longer than 
its sidereal period. This will be evident from an examina- 
tion of Fig. 147. ^ is the position of the sun, E that of 
the earth, and P that of the planet at inferior conjunc- 
tion. Before the planet can be in inferior conjunction 
again, it must pass entirely around its orbit, and overtake 
the earth, which has in the mean time passed on in its orbit 
to E'. 

While the earth is passing from E to E\ the planet 
passes entirely around its orbit, and from P to P' in addition. 




ASTRONOMY. I33 

Now the arc PF is just equal to the arc E E r : hence the 
planet has to pass over the same arc that the earth does, 
and 360 more. In other words, the planet has to gain 
360 on the earth. 

The synodical period of the planet is found by direct 
observation. 

129. The Length of the Sidereal Period. — The length of 
the sidereal period of an inferior planet may be found by the 
following computation : — 

Let a denote the synodical period of the planet, 

Let b denote the sidereal period of the earth, 

Let x denote the sidereal period of the planet. 

^6o° 
Then ^-7— = the daily motion of the earth, , " 

360 
And = the daily motion of the planet, 

^6o° ^6o° 
And ^— — ^-7— = the daily gain. of the planet; 

^6o° 
Also = the daily gain of the planet: 

tt 360 360 360 

Hence ^ ^-v— = . 

x a 

Dividing by 360 , we have - — -7 — — ; 

Clearing of fractions, we have ab — ax = bx : 
Transposing and collecting, we have (a + b)x = ab ; 

Therefore x — — - — ,. 
a -f- b 

130. The Relative Distance of an Inferior Planet. — By the 
relative distance of a planet, we mean its distance from the sun 
compared with the earth's distance from the sun. The relative 
distance of an inferior planet may be found by the following 
method : — 

Let V, in Fig. 148, represent the position of Venus at its 
greatest elongation from the sun, 6* the position of the sun, 
and E that of the earth. The line E V will evidently be tan- 
gent to a circle described about the sun with a radius equal 
to the distance of Venus from the sun at the time of this great- 



134 



ASTRONOMY. 



est elongation. Draw the radius S V and the line SE. Since 
SV\% a radius, the angle at V is a right angle. The angle 
at E is known by measurement, and the angle at 6* is equal to 
g o _ t h e angle E. In the right-angled triangle E VS, we then 
know the three angles, and we wish to find the ratio of the 
side SVto the side SE. 

The ratio of these lines may be found by trigonometrical 
computation as follows : — 

VS : ES= sin SEV\ i. 

Substitute the value of the sine of SE V and we have 

VS: ES= .723 : 1. 

Hence the relative distances of Venus and of the earth from 
the sun are .723 and 1. 

Superior Planets. 

131. The Superior Planets. — The superior planets are 
those which lie beyond the earth. They are Afars, the 
Asteroids, Jupiter, Saturn, Uranus, and Neptune. 

132. Apparent Motion of a Superior Planet. — In order to 
deduce the apparent motion of a superior 
planet from the real motions of the earth 
and planet, let S (Fig. 149) be the place 
of the sun; 1, 2, 3, etc., the orbit of the 
earth ; a, b, c, etc., the orbit of Mars : and 
CGL a part of the starry firmament. Let 
the orbit of the earth be divided into 
twelve equal parts, each described in one 
month; and let ab, be, cd, etc., be the 
spaces described by Mars in the same 
time. Suppose the earth to be at the 
point 1 when Mars is at the point a, Mars 
will then appear in the heavens in the 
direction of 1 a. When the earth is at 
3, and Mars at c. he will appear in the 
heavens at C. When the earth arrives 

at 4, Mars will arrive at d. and will appear in the heavens at D. 
While the earth moves from 4 to 5 and from 5 to 6, Mars will 




ASTRONOMY. 



135 




appear to have advanced among the stars from D to E ancj, 

from E to F, 

in the direction 

from west to 

east. During 

the motion of 

the earth from 6 

to 7 and from 7 

to 8, Mars will 

appear to go 

backward from 

F to G and 

from G to ff 9 

in the direction 

from east to 

west. During 

the motion of 

the earth from 8 

to 9 and from 9 

to 10, Mars will 

appear to advance from H to / and from / to K,m the direction 

from west to east, 
and the motion will 
continue in the 
same direction until 
near the succeeding 
opposition. 

The apparent mo- 
tion of a superior 
planet projected on 
the heavens is thus 
seen to be similar 
to that of an infe- 
rior planet, except 
that, in the latter 
case, the retrogres- 
Flg - I5 °- sion takes place near 

inferior conjunction, and in the former it takes place near 

opposition. 





136 



ASTRONOMY. 



133. Aspects of a Superior Planet. — The four aspects of 
a superior planet are shown in Fig. 150, in which S is the 
position of the sun, E that of the earth, and P that of the 
planet. 

When the planet is on the opposite side of the earth tc 
the sun, as at P, it is said to be in opposition. The sur 




and the planet will then appear in opposite parts of the 
heavens, the sun appearing at C, and the planet at A. 

When the planet is on the opposite side of the sun to 
the earth, as at P", it is said to be in superior conjunction. 
It will then appear in the same part of the heavens as the 
sun, both appearing at C. 

When the planet is at P f and P'" 9 so that a line drawn 
from the earth through the planet will make a right angle 
with a line drawn from the earth to the sun, it is said to be 



ASTRONOMY. 



137 



in quadrature. At P r it is in its western quadrature, and at 
P"' in its eastern quadrature. 

134. Phases of a Superior Planet. — Mars is the only 
one of the superior planets that has appreciable phases. 
At quadrature, as will appear from Fig. 151, Mars does not 
present quite the same side to the earth as to the sun : 
hence, near these parts of its orbit, the planet appears slightly 
gibbous. Elsewhere in its orbit, the planet appears fall. 

All the other superior planets are so far away from the 
sun and earth, that the sides which they turn towards the 
sun and the earth in every part of their orbit are so nearly 
the same, that no change in the form of their disks can be 
detected. i*_ 

135. The Syno die a 1 Pe rio d 
of a Superior Planet. — Dur- 
ing a synodical period of a 
superior planet the earth must 
gain one revolution, or 360°, 
on the planet, as will be evi- 
dent from an examination of 
Fig. 152, in which 5 repre- 
sents the sun. E the earth, 
and P the planet at opposi- 
tion. Before the planet can 




Fig. 152. 

be in opposition again, 



me 

earth must make a complete revolution, and overtake the 
planet, which has in the mean time passed on from Pto P\ 

In the case of most of the superior planets the synodical 
period is shorter than the sidereal period : but in the case 
of Mars it is longer, since Mars makes more than a com- 
plete revolution before the earth overtakes it. 

The synodical period of a superior planet is found by 
direct observation. 

136. The Sidereal Period of a S7cperior Planet. — The 
sidereal period of a superior planet is found by a method of 
computation similar to that for finding the sidereal period of 
an inferior planet : — 



138 



ASTRONOMY. 



Let a denote the synodical period of the planet. 

Let b denote the sidereal period of the earth, 

Let x denote the sidereal period of the planet. 

^6o° 
Then will ^—j— — daily motion of the earth, 



And 
Also 
But 
Hence 



b 
36o° 

x 
360 

b 

^6o° 

= daily °:ain of the earth : 

a J & • 



daily motion of the planet ; 
360 



= daily gain of the earth. 



360 
b " 



360 
x 



360 
a 
1 1 1 

b x a 
ax — ab — bx 
(a — b)x — ab 
ab 



a — b'- 
137. The Relative Distance of a Superior Planet. — Let 




S, e, and m, in Fig. 153, represent the relative positions of the 
sun, the earth, and Mars, when the latter planet is in opposi- 
tion. Let E and M represent the relative positions of the 
earth and Mars the day after opposition. At the first observa- 
tion Mars will be seen in the direction em A, and at the second 
observation in the direction EM A. 

But the fixed stars are so distant, that if a line, eA, were 
drawn to a fixed star at the first observation, and a line, EB, 
drawn from the earth to the same fixed star at the second 



ASTRONOMY. 1 39 

observation, these two lines would be sensibly parallel ; that is, 
the fixed star would be seen in the direction of the line eA at 
the first observation, and in the direction of the line EB, par- 
allel to eA, at the second observation. But if Mars were seen 
in the direction of the fixed star at the first observation, it would 
appear back, or west, of that star at the second observation by 
the angular distance BEAj that is, the planet would have 
retrograded that angular distance. Now, this retrogression of 
Mars during one day, at the time of opposition, can be meas- 
ured directly by observation. This measurement gives us the 
value of the angle BE Aj but we know the rate at which both 
the earth and Mars are moving in their orbits, and from this 
we can easily find the angular distance passed over by each in 
one day. This gives us the angles ESA and MSA. We can 
now find the relative length of the lines MS and ES (which 
represent the distances of Mars and of the earth from the sun), 
both by construction and by trigonometrical computation. 

Since EB and eA are parallel, the angle EAS is equal 
to BE A. 

SEA = 180 - (ESA + EAS) 

ESJlf= ESA -MSA 

EMS = 180 - {SEA + ESM). 

We have then 

MS : ES = sin SEA : sin EMS. 
Substituting 1 the values of the sines, and reducing: the ratio 
to its lowest terms, we have 

MS : ES= 1.524 : 1. 

Thus we find that the relative distances of Mars and the 
earth from the sun are 1.524 and 1. By the simple observation 
of its greatest elongation,- we are able to determine the relative 
distances of an inferior planet and the earth from the sun : 
and, by the equally simple observation of the daily retrogres- 
sion of a superior planet, we can find the relative distances of 
such a planet and the earth from the sun. 



140 



ASTRONOMY. 



IV. THE SUN. 
I. MAGNITUDE AND DISTANCE OF THE SUT 

138. The Volume of the Sun. — The apparent diameter 
of the sun is about 32', being a little greater than that of 
the moon. The real diameter of the sun is 866,400 miles, 
or about a hundred and nine times that of the earth. 

As the diameter of the moon's orbit is only about 480,000 

miles, or some 
sixty times the 
diameter of the 
earth, it follows 
that the diameter 
of the sun is 
nearly double that 
of the moon's 
orbit : hence, were 
the centre of the 
sun placed at the 
centre of the earth, 
the sun would 
completely fill the 
moon's orbit, and 
reach nearly as 

far beyond it in every direction as it is from the earth to 

the moon. The circumference of the sun as compared with 

the moon's orbit is shown in Fig. 154. 

The volume of the sun is 1,305,000 times that of th 

earth. 

139. The Mass of the Sun. — The sun is much less 
dense than the earth. The mass of the sun is only 330,000 
times that of the earth, and its density only about a fourth 
that of the earth. 




Fig. 154. 



To find the mass of the sun, we first ascertain the distance 



ASTRONOMY. 



141 



the earth would draw the moon towards itself in a given time, 
were the moon at the distance of the sun, and then form the 
proportion : as the distance the earth would draw the moon 
towards itself is to the distance that the sun draws the earth 
towards itself in the same time, so is the mass of the earth to 
the mass of the sun. 

Although the mass of the sun is over three hundred thou- 




sand times that of the earth, the pull of gravity at the surface 
of the sun is only about twenty-eight times as great as at 
the surface of the earth. This is because the distance from 
the surface of the sun to its centre is much greater than 
from the surface to the centre of the earth. 

140. Size of the Sun Co?npa?-ed with that of the Planets, 
— The size of the sun compared with that of the larger 



142 



ASTRONOMY. 



planets is shown in Fig. 155. The mass of the sun is more 
than seven hundred and fifty times that of all of the planets 
and moons in the solar system. In Fig. 156 is shown the 




Fig. 156. 

apparent size of the sun as seen from the different planets. 
The apparent diameter of the sun decreases as the distance 
from it increases, and the disk of the sun decreases as the 
square of the distance from it increases. 

141. The Distance of the Sun.— The mean distance 



ASTRONOMY. I43 

of the sun from the earth is about 92,800,000 miles. Owing 
to the eccentricity of the earth's orbit, the distance of the 
sun varies somewhat ; being about 3,000,000 miles less in 
January, when the earth is at perihelion, than in June, when 
the earth is at aphelion. 

" But, though the distance of the sun can easily be stated in 
rigures, it is not possible to give any real idea of a space so 
enormous : it is quite beyond our power of conception. If one 
were to try to walk such a distance, supposing that he could 
walk four miles an hour, and keep it up for ten hours every day, 
it would take sixty-eight years and a half to make a single 
million of miles, and more than sixty-three hundred years to 
traverse the whole. 

" If some celestial railway could be imagined, the journey to 
the sun, even if our trains ran sixty miles an hour day and night 
and without a stop, would require over a hundred and seventy- 
five years. Sensation, even, would not travel so far in a human 
lifetime. , To borrow the curious illustration of Professor Men- 
denhall, if we could imagine an infant with an arm long enough 
to enable him to touch the sun and burn himself, he would die 
of old age before the pain could reach him ; since, according 
to the experiments of Helmholtz and others, a nervous shock 
is communicated only at the rate of about a hundred feet per 
second, or 1,637 miles a day, and would need more than a hun- 
dred and fifty years to make the journey. Sound would do it 
in about fourteen years, if it could be transmitted through celes- 
tial space ; and a cannon-ball in about nine, if it were to move 
uniformly with the same speed as when it left the muzzle of 
the gun. If the earth could be suddenly stopped in her orbit, 
and allowed to fall unobstructed toward the sun, under the 
accelerating influence of his attraction, she would reach the 
centre in about four months. I have said if she could be 
stopped; but such is the compass of her orbit, that, to make 
its circuit in a year, she has to move nearly nineteen miles a 
second, or more than fifty times faster than the swiftest rifle- 
ball ; and, in moving twenty miles, her path deviates from per- 
fect straightness by less than an eighth of an inch. And yet, 
over all the circumference of this tremendous orbit, the sun 



144 



ASTRONOMY. 



exercises his dominion, and every pulsation of his surface 
receives its response from the subject earth." 1 

142. Method of Finding the Surfs Distance. — There are 
several methods of finding the sun's distance. The simplest 
method is that of finding the actual distance of one of the 
nearer planets by observing its displacement in the sky as seen 
from widely separated points on the earth. As the relative 
distances of the planets from each other and from the sun are 
well known, we can easily deduce the actual distance of the 
sun if we can find that of any of the planets. The two planets 
usually chosen for this method are Mars and Venus. 

(1) The displacement of Mars in the sky, as seen from two 
observatories which differ considerably in latitude, is, of course, 
greatest when Mars is nearest the earth. Now, it is evident 
than Mars will be nearer the earth when in opposition than 




F ; g. 157- 

when in any other part of its orbit ; and the planet will be least 
distant from the earth when it is at its perihelion point, and the 
earth is at its aphelion point, at the time of opposition. This 
method, then, can be used to the best advantage, when, at the 
time of opposition, Mars is near its perihelion, and the earth 
near its aphelion. These favorable oppositions occur about 
once in fifteen years, and the last one was in 1877. 

Suppose two observers situated at N' and S' (Fig. 157), near 
the poles of the earth. The one at N' would see Mars in the 
sky at JV, and the one at S f would see it at S. The displace- 
ment would be the angle NMS. Each observer measures 
carefully the distance of Mars from the same fixed star near 
it. The difference of these distances gives the displacement 
of the planet, or the angle NMS. These observations were 
made with the greatest care in 1877. 

1 Professor C. A. Young : The Sun. 



ASTRONOMY. 



145 



(2) Venus is nearest the earth at the time of inferior con- 
junction ; but it can then be seen only in the daytime. It is, 
therefore, impossible to ascertain the displacement of Venus, 
as seen from different stations, by comparing her distances 
from a fixed star. Occasionally, at the time of inferior con- 
junction, Venus passes directly across the sun's disk. The 
last of these transits of Venus occurred in 1874, and the next 
will occur in 1882. It will then be over a hundred years before 
another will occur. 

Suppose two observers, A and B (Fig. 158), near the poles 
of the earth at the time of a transit of Venus. The observer 
at A would see Venus crossing the sun at V& and the one at 
B would see it crossing at l\. Any observation made upon 




Fig. 158. 

Venus, which would give the distance and direction of Venus 
from the centre of the sun, as seen from each station, would 
enable us to calculate the angular distance between the tw r o 
chords described across the sun. This, of course, would give 
the displacement of Venus on the sun's disk. This method was 
first employed at the last transits of Venus which occurred 
before 1874: namely, those of 1761 and 1769. 

There are three methods of observation employed to ascer- 
tain the apparent direction and distance of Venus from the 
centre of the sun, called respectively the contact method, the 
micrometric method, and the photographic 7nethod. 

(a) In the contact method, the observation consists in noting 
the exact time when Venus crosses the sun's limb. To ascer- 



146 



ASTRONOMY. 



tain this it is necessary to observe the exact time of external 
and internal contact. This observation, though apparently 
simple, is really very difficult. With reference to this method 
Professor Young says, — 

" The difficulties depend in part upon the imperfections of 
optical instruments and the human eye, partly upon the essen- 
tial nature of light leading to what is known as diffraction, and 
partly upon the action of the planet's atmosphere. The two 
first-named causes produce what is called irradiation, and oper- 
ate to make the apparent diameter of the planet, as seen on the 
solar disk, smaller than it really is ; smaller, too, by an amount 
which varies with the size of the telescope, the perfection of 
its lenses, and the tint and brightness of the sun's image. 

The edge of the plan- 
et's image is also ren- 
dered slightly hazy and 
indistinct. 

" The planet's at- 
mosphere also causes 
its disk to be sur- 
rounded by a narrow 
ring of light, which 
Fig - I5 9- becomes visible long 

before the planet touches the sun, and, at the moment of inter- 
nal contact, produces an appearance, of which the accompany- 
ing figure is intended to give an idea, though on an exaggerated 
scale. The planet moves so slowly as to occupy more thari 
twenty minutes in crossing the sun's limb ; so that even if the 
planet's edge were perfectly sharp and definite, and the sun's 
limb undistorted, it would be very difficult to determine the 
precise second at which contact occurs. But, as things are, 
observers with precisely similar telescopes, and side by side, 
often differ from each other five or six seconds ; and, where the 
telescopes are not similar, the differences and uncertainties are 
much greater. . . . Astronomers, therefore, at present are 
pretty much agreed that such observations can be of little value 
in removing the remaining uncertainty of the parallax, and are 
disposed to put more reliance upon the micrometric and photo- 
graphic methods, which are free from these peculiar difficulties, 




ASTRONOMY. 147 

though, of course, beset with others, which, however, it is 
hoped will prove less formidable." 

(b) Of the micrometric method, as employed at the last 
transit, Professor Young speaks as follows : — 

" The micrometric method requires the use of a heliometer, 
— an instrument common only in Germany, and requiring much 
skill and practice in its use in order to obtain with it accurate 
measures. At the late transit, a single English party, two or 
three of the Russian parties, and all five of the German, were 
equipped with these instruments ; and at some of the stations 
extensive series of measures were made. None of the results, 
however, have appeared as yet ; so that it is impossible to say 
how greatly, if at all, this method will have the advantage in 
precision over the contact observations." 

(c) The following observations, with reference to the photo- 
graphic method, are also taken from Professor Young : — 

" The Americans and French placed their main reliance upon 
the photographic method, while the English and Germans also 
provided for its use to a certain extent. The great advantage 
of this method is, that it makes it possible to perform the 
necessary measurements (upon whose accuracy every thing 
depends) at leisure after the transit, without hurry, and with all 
possible precautions. The field-work consists merely in obtain- 
ing as many and as good pictures as possible. A principal 
objection to the method lies in the difficulty of obtaining good 
pictures, i.e., pictures free from distortion, and so distinct and 
sharp as to bear high magnifying power in the microscopic 
apparatus used for their measurement. The most serious diffi- 
culty, however, is involved in the accurate determination of the 
scale of the picture ; that is, of the number of seconds of arc 
corresponding to a linear inch upon the plate. Besides this, 
we must know the exact Greenwich time at which each picture 
is taken, and it is also extremely desirable that the orientation 
3f the picture should be accurately determined ; that is, the 
lorth and south, the east and west points of the solar image on 
:he finished plate. There has been a good deal of anxiety lest 
he image, however accurate and sharp when first produced, 
should alter, in course of time, through the contraction of the 
:ollodion film on the glass plate ; but the experiments of 



148 



ASTRONOMY. 






Ruth erf urd, Huggins. and Paschen, seem to show that this 
danger is imaginary. . . . The Americans placed the photo- 
graphic telescope exactly in line with a meridian instrument, 
and so determined, with the extremest precision, the direction 
in which it was pointed. Knowing this and the time at which 
any picture was taken, it becomes possible, with the help of 
the plumb-line image, to determine precisely the orientation of 
the picture, — an advantage possessed by the American pictures 
alone, and making their value nearly twice as great as otherwise 
it would have been. 

" The figure below is a representation of one of the Ameri- 
can photographs re- 
duced about one- 
half. F is the 
image of Venus, 
which, on the actual 
plate, is about a 
seventh of an inch 
in diameter: a a' is 
the image of the 
plumb-line. The 

centre of the reticle 
is marked with a 
cross. " 

The English pho- 
tographs proved tc 
be of little value, 
and the results oi 
the measurements and calculations upon the American pictures 
have not yet been published. There is a growing apprehension 
that no photographic method can be relied upon. 

The most recent determinations by various methods indi- 
cate that the sun's distance is such that his parallax is abou' 
eighty-eight seconds. This would make the linear value of 
a second at the surface of the sun about four hundred anc 
fifty miles, 

























9 V 








































£ 


■ass 


















\ 












M. 


i 



















































Fig. 160. 



PLATE I. 




ASTRONOMY. I49 

II. PHYSICAL AND CHEMICAL CONDITION OF 

THE SUN. 

Physical Condition of the Sun. 

143. The Sun Composed mainly of Gases. — It is now 
generally believed that the sun is mainly a ball of gas, or 
vapor, powerfully condensed at the centre by the weight 
of the superincumbent mass, but kept from liquefying by its 
exceedingly high temperature. 

The gaseous interior of the sun is surrounded by a layer 
of luminous clouds, which constitutes its visible surface, and 
which is called its photosphere. Here and there in 'the 
photosphere are seen dark spots, which often attain an 
immense- magnitude. 

These clouds float in the solar atmosphere, which extends 
some distance beyond them. 

TheJuminous surface of the sun is surrounded by a rose- 
colored stratum of gaseous matter, called the chromosphere. 
Here and there great masses of this chromospheric matter 
rise high above the general level. These masses are called 
pro mine ?ices. 

Outside of the chromosphere is the corona, an irregular 
halo of faint, pearly light, mainly composed of filaments 
and streamers, which radiate from the sun to enormous dis- 
tances, often more than a million of miles. 

In Fig. 161 is shown a section of the sun, according to 
Professor Young. 

The accompanying lithographic plate gives a general view 
of the photosphere with its spots, and of the chromosphere 
and its prominences. 

144. The Temperatiu'e of the Sun. — Those who have 
investigated the subject of the temperature of the sun have 
come to very different conclusions ; some placing it as high 
as four million degrees Fahrenheit, and others as low as 
ten thousand degrees. Professor Young thinks that Rosetti's 



ISO 



ASTRONOMY. 






estimate of eighteen thousand degrees as the effective tem- 
perature of the sun's surface is probably not far from 
correct. By this is meant the temperature that a uniform 
surface of lampblack of the size of the sun must have in 
order to radiate as much heat as the sun does. The most 
intense artificial heat does not exceed four thousand degrees 
Fahrenheit. 




145. The Amount of Heat Radiated by the Sun.—h 
unit of heat is the amount of heat required to raise a 
pound of water one degree in temperature. It takes about 
a hundred and forty-three units of heat to melt a pound of 
ice without changing its temperature. A cubic foot of ice 
weighs about fifty-seven pounds. According to Sir William 
Herschel, were all the heat radiated by the sun concentrated 



ASTRONOMY. 



151 



on a cylinder of ice forty-five miles in diameter, it would 
melt it off at the rate of about a hundred and ninety thou- 
sand miles a second. 

Professor Young gives the following illustration of the 
energy of solar radiation : " If we could build up a solid 
column of ice from the earth to the sun, two miles and a 
quarter in diameter, spanning the inconceivable abyss of 
ninety-three million miles, and if then the sun should con- 
centrate his power upon it, it would dissolve and melt, not 
in an hour, nor a minute, but in a single second. One 
swing of the pendulum, and it would be water ; seven more, 
and it would be dissipated in vapor." 

This heat would be 
sufficient to melt a layer 
of ice nearly fifty feet 
thick all around the sun 
in a minute. To develop 
this heat would require 
the hourly consumption 
of a layer of anthracite 
coal, more than sixteen 
feet thick, over the entire 
surface of the sun ; and the mechanical equivalent of this 
heat is about ten thousand horse-power on every square 
foot of the sun's surface. 

146. The Brightness of the Sun's Surface, — The sun's 
surface is a hundred and ninety thousand times as bright as 
a candle-flame, a hundred and forty-six times as bright as 
the calcium-light, and about three times and a half as bright 
as the voltaic arc. 

The sun's disk is much less bright near the margin than 
near the centre, a point on the limb of the sun being only 
about a fourth as bright as one near the centre of the disk. 
This diminution of brightness towards the margin of the 
disk is due to the increase in the absorption of the solar 




Fig. 162. 



152 



ASTRONOMY. 



atmosphere as we pass from the centre towards the margin 
of the sun's disk ; and this increased absorption is due to 
the fact, that the rays which reach us from near the margin 
have to traverse a much greater thickness of the solar 
atmosphere than those which reach us from the centre of 
the disk. This will be evident from Fig. 162, in which the 
arrows mark the paths of rays from different parts of the 
solar disk. 

The Spectroscope, 

147. The Spectroscope as an Astronomical Instrument 
— The spectroscope is now continually employed in the 
study of the physical condition and chemical constitution 




Fig. 163. 

of the sun and of the other heavenly bodies. It has 
become almost as indispensable to the astronomer as the 
telescope. 

148. The Dispersion Spectroscope. — The essential parts 
of the dispersion spectroscope are shown in Fig. 163. 
These are the collimator tube, the prism, and the telescope. 
The collimator tube has a narrow slit at one end, through 
which the light to be examined is admitted, and some- 
where within the tube a lens for condensing the light. The 
light is dispersed on passing through the prism : it then 
passes through the objective of the telescope, and forms 



ASTRONOMY. 



153 



within the tube an image of the spectrum, which is exam- 
ined by means of the eye-piece. The power of the spec- 
troscope is increased by increasing the number of prisms, 
which are arranged so 
that the light shall pass 
through one after an- 
other in succession. 
Such an arrangement 
of prisms is shown in 
Fig. 164. One end 
of the collimator tube 
is seen at the left, and 
one end of the tele- 
scope at the right. 
Sometimes the prisms 
are made long, and 
the light is sent twice 
through the same train 
of prisms, once through 
the lower, and once Flg - ID4 ' 

through the upper, half of the prisms. This is accom- 
plished by placing a rectangular prism against the last 

prism of the train, 
as shown in Fig. 

165. 

149. The Mi- 
crometer Scale. — 
Various devices are 
employed to obtain 
an image of a mi- 
crometer scale in 
the tube of the 





Fig. 165. 



telescope beside that of the spectrum. 

One of the simplest of these methods is shown in 
Fig. 166. A is the telescope, B the collimator, and C the 



154 



ASTRONOMY. 



micrometer tube. The opening at the outer end of C con- 
tains a piece of glass which has a micrometer scale marked 
upon it. The light from the candle shines through this 




Fig. 166. 



glass, falls upon the surface of the prism P. and is thence 
reflected into the telescope, where it forms an enlarged 




:;7 V ^ : SltfL 




image of the micrometer scale alongside the image of the 
spectrum. 

150. The Comparison of Spectra. — In order to com- 



ASTRONOMY. 



155 



pare two spectra, it is desirable to be able to see them 

side by side in the telescope. The images of two spectra 

may be obtained side by side in the telescope tube by the 

use of a little rectangular prism, which covers one-half 

of the slit of the collimator tube, as shown in Fig. 167. 

The light from one source is admitted directly through 

the uncovered half of the slit, while the light from the 

other source is sent 

through the covered 

portion of the slit 

by reflection from 

the surface of the 

rectangular prism. Fi s- l68 - 

This arrangement and its action will be readily understood 

from Fig. 167. 

151. Direct -Vision Spectroscope, — A beam of light may 
be dispersed, without any ultimate deflection from its 
course, by combining prisms of crown and flint glass with 
equal refractive, but unequal dispersive powers. Such a 
combination of prisms is called a direct-vision combination. 





Fig. i6q. 



One of three prisms is shown in Fig. 168, and one of five 
prisms in Fig. 169. 

A direct-vision spectroscope (Fig. 1 70) is one in which 
a direct-vision combination of prisms is employed. C is 
the collimator tube, P the train of prisms, F the telescope, 
and r the comparison prism. 

152. The Telespectroseope. — The spectroscope, when 
used for astronomical work, is usually combined with a 



156 



ASTRONOMY. 



telescope. The compound instrument is called a tekspec- 
troscope. The spectroscope is mounted at the end of the 
telescope in such a way that the image formed by the 




Fig. 170. 

object-glass of the telescope falls upon the slit at the end 
of the collimator tube. A telespectroscope of small dis- 
persive power is shown in Fig. 171; a being the object- 
glass of the telescope, cc the tube of the telescope, and 




Fig. 171- 



e the comparison prism at the end of the collimator tube. 
A more powerful instrument is shown in Fig. 172. A is 
the telescope, C the collimator tube of the spectroscope, 



ASTRONOMY. 



iS7 



P the train of prisms, and E the telescope tube. Fig. 173 
shows a still more powerful spectroscope attached to the 
great Newall refractor (18). 




Fig. 172. 



153. The Diffraction Spectroscope. — A diffraction spec- 
troscope is one in which the spectrum is produced bv 




Fig- 173. 



reflection of the light from a finely ruled surface, or grating, 
as it is called, instead of by dispersion in passing through a 



I58 ASTRONOMY. 

prism. The essential parts of this instrument are shown 
in Fig 174. This spectroscope may be attached to the 
telescope in the same manner as the dispersion spectroscope. 
When the spectroscope is thus used, the eye-piece of the 
telescope is removed. 

Spectra. 

154. Continuous Spectra. — Light from an incandescent 
solid or liquid which has suffered no absorption in the 
medium which it has traversed gives a spectrum consisting 
of a continuous colored band, in which the colors, from 
the red to the violet, pass gradually and imperceptibly into 




Fig. 174. 

one another. The spectrum is entirely free from either light 
or dark lines, and is called a continuous spectrum. 

155. Bright-Lined Spectra. — Light from a luminous gas 
or vapor gives a spectrum composed of bright lines sepa- 
rated by dark spaces, and known as a bright-lined spec- 
trum. It has been found that the lines in the spectrum of 
a substance in the state of a gas or vapor are the most 
characteristic thing about the substance, since no two vapors 
give exactly the same lines : hence, when we have once 
become acquainted with the bright-lined spectrum of any 
substance, we can ever after recognize that substance by 
the spectrum of its luminous vapor. Even when several 
substances are mixed, they may all be recognized by the 
bright-lined spectrum of the mixture, since the lines of 



ASTRONOMY. 



159 



all the substances will be present in the spectrum of the 
mixture. This method of identifying substances by their 
spectra is called spectrum analysis. 

The bright-lined spectra of several substances are given 
in the frontispiece. The number of lines in the spectra of 
the elements varies greatly. The spectrum of sodium is one 
of the simplest, while that of iron is one of the most com- 
plex. The latter contains over six hundred lines. Though 
no two vapors give identical spectra, there are many cases 
in which one or more of the spectral lines of one element 
coincide in position with lines of other elements. 

156. Methods of rendering Gases and Vapors Luminous. — 
In order to study the spectra of vapors and gases it is neces- 
sary to have some means of converting solids and liquids into 
vapor, and also of rendering 
the vapors and gases lumi- 
nous. There are four meth- 
ods of obtaining luminous 
vapors and gases in common 
use. 

(1) By means of the Bun- 
sen Flame. — This is a very 
hot but an almost non- 
luminous flame. If any 
readily volatilized substance, 
such as the compounds of 
sodium, calcium, strontium, 
etc., is introduced into this 
flame on a fine platinum wire, 
it is volatilized in the flame, 
and its vapor is rendered 




Fig. 175. 



luminous, giving the flame its own peculiar color. The flame 
thus colored may be examined by the spectroscope. The 
arrangement of the flame is shown in Fig. 175. 

(2) By means of the Voltaic Arc. — An electric lamp is 
shown in Fig. 176. When this lamp is to be used for obtain- 
ing luminous vapors, the lower carbon is made larger than the 



i6o 



ASTRONOMY. 



upper one, and hollowed out at the top into a little cup. The 
substance to be volatilized is placed in this cup, and the current 
is allowed to pass. The heat of the voltaic arc is much more 
intense than that of the Bunsen flame : hence substances that 

cannot be volatil- 
ized in the flame 
are readily vola- 
tilized in the arc, 
and the vapor 
formed is raised 
to a very high 
temperature. 

(3) By means 
of the Spark 
from an Indue- 
tion Coil. — The 
arrangement of 
the coil for ob- 
taining luminous 
vapors is shown 
in Fig. 177. 

The terminals 
of the coil be- 
tween which the 
spark is to pass 
are brought quite 
close together. 
When we wish 
to vaporize any 
metal, as iron, 
the terminals are 
made of iron. 
On the passage 
of the spark, a 
little of the iron at the ends of the terminals is evaporated; 
and the vapor is rendered luminous in the space traversed by 
the spark. A condenser is usually placed in the circuit. With 
the coil, the temperature may be varied at pleasure ; and the 
vapor may be raised even to a higher temperature than with 




176. 



ASTRONOMY. 



161 



the electric lamp. To obtain a low temperature, the coil is 
used without the condenser. By using a larger and larger 
condenser, the temperature may be raised higher and higher. 

By means of the induction coil we may also heat gases to 
incandescence. It is only necessary to allow the spark to 
pass through a space filled with the gas. 

(4) By means of a Vacuum Tube. — The form of the vacuum 
tube commonly used for this purpose is shown in Fig. 178. 
The gas to be examined, and which is contained in the tube 
has very slight density ; but upon the passage of the discharge 
from an induction 
coil or a Holtz 
machine, through 
the tube, the gas 
in the capillary 
part of the tube 
becomes heated to 
a high tempera- 
ture, and is then 
quite brilliant. 

157. Reversed 
Spectra. — If the 
light from an in- 
candescent cylin- 
der of lime, or 
from the incan- 
descent point of an electric lamp, is allowed to pass 
through luminous sodium vapor, and is then examined with 
a spectroscope, the spectrum will -be found to be a bright 
spectrum crossed by a single dark line in the position of 
the yellow line of the sodium vapor. The spectrum of 
sodium vapor is reversed, its bright lines becoming dark 
and its dark spaces bright. With a spectroscope of any 
considerable power, the yellow line of sodium vapor is 
resolved into a double line. With a spectroscope of the 
same power, the dark sodium line of the reversed spectrum 
is seen to be a double line. 




l62 



ASTRONOMY. 



It is found to be generally true, that the spectrum of the 
light from an incandescent solid or liquid which has passed 
through a luminous vapor on its way to the spectroscope 
is made up of a bright ground crossed by dark lines ; there 

being a dark line for every bright line that the 

vapor alone would give. 

158. Explanation of Reversed Spectra. — It 
has been found that gases absorb and quench rays 
of the same degree of refrangibility as those 
which they themselves emit, and no others. 
When a solid is shining through a luminous 
vapor, this absorbs and quenches those rays from 
the solid which have the same degrees of refrangi- 
bility as those which it is itself emitting: hence 
the lines of the spectrum receive light from the 
vapor alone, while the spaces between the lines 
receive light from the solid. Now, solids and 
liquids, when heated to incandescence, give a very 
much brighter light than vapors and gases at the 
same temperature : hence the lines of a reversed 
spectrum, though receiving light from the vapor 
or gas, appear dark by contrast. 

159. Effect of Increasing the Power of the 
Spectroscope upon the Brilliancy of a Spectrum. 
— An increase in the power of a spectroscope 
diminishes the brilliancy of a continuous spec- 
trum, since it makes the colored band longer, and 
therefore spreads the light out over a greater 
extent of surface; but, in the case of a bright- 
lined spectrum, an increase of power in the spec- 
troscope produces scarcely any alteration in the 

brilliancy of the lines, since it merely separates the lines far- 
ther without making the lines themselves any wider. In the 
case of a reversed spectrum, an increase of power in the spec- 
troscope dilutes the light in the spaces between the lines 
without diluting that of the lines : hence lines which appear 
dark in a spectroscope of slight dispersive power may appear 
bright in an instrument of great dispersive power. 



Fig. 178. 






ASTRONOMY. 



163 



\j 



green \z_ '" 



160. Change of the Spectrum with the Density of the Lu- 
minous Vapor. — It has been found, that, as the density of a 
luminous vapor is diminished, the lines in its spectrum be- 
come fewer and fewer, till 
they are finally reduced to 
one. On the other hand, an 
increase of density causes 
new lines to appear in the 
spectrum, and the old lines to 
become thicker. 

161. Change of the Spec* ^el/ow 
trum with the Teniperature 
of the Lui?iinous Vapor. — 
It has also been found that 
the appearance of a bright- 
lined spectrum changes con- ^ 
siderably with the tempera- 
ture of the luminous vapor. 
In some cases, an increase 
of temperature changes the Schuo 
relative intensities of the 
lines ; in other cases, it causes 
new lines to appear, and old 
lines to disappear. 

In the case of a com- 
pound vapor, an increase 
of temperature causes the 
colored bands (which are 
peculiar to the spectrum of 
the compound) to disappear, 
and to be replaced by the 
spectral lines of the ele- 
ments of which the com- 
pound is made up. The 
heat appears to dissociate Flg- I79< 

the compound; that is, to resolve it into its constituent 
elements. In this case, each elementary vapor would give its 
own spectral lines. As the compound is not completely dis- 
sociated at once, it is possible, of course, for one or more of 



T£&&* 




164 



ASTRONOMY. 



the spectral lines of the elementary vapors to co-exist in the 
spectrum with the bands of the compound. 

It has been found, that, in some cases, the spectra of the 
elementary gases change with the temperature of the gas ; and 
Lockyer thinks he has discovered conclusive evidence, in the 
spectra of the sun and stars, that many of the substances 
regarded as elementary are really resolved into simpler sub- 
stances by the intense heat of the sun ; in other words, that our 
so-called elements are really compounds. 



Chemical Constitution of the Sun. 

162. The Solar Spectrum. — The solar spectrum is 
crossed transversely by a great number of fine dark lines, 
and hence it belongs to the class of reversed spectra. 




Fig. 180. 



These lines were first studied and mapped by Fraun- 
hofer, and from him they have been called Fraunhofer's 
lines. 

A reduced copy of Fraunhofer's map is shown in Fig. 179. 
A few of the most prominent of the dark solar lines are 
designated by the letters of the alphabet. The other lines are 
usually designated by the numbers at which they are found on 
the scale which accompanies the map. This scale is usually 
drawn at the top of the map, as will be seen in some of the 
following diagrams. The two most elaborate maps of the solar 
spectrum are those of Kirchhoff and Angstrom. The scale on 
KirchhofFs map is an arbitrary one, while that of Angstrom is 
based upon the wave-lengths of the rays of light which would 
fall upon the lines in the spectrum. 

The appearance of the spectrum varies greatly with the 



ASTRONOMY. 



165 



power of the spectroscope employed. Fig. 180 shows a por- 
tion of the spectrum as it appears in a spectroscope of a single 
prism; while Fig. 181 shows the b group of lines alone, as they 
appear in a powerful diffraction spectroscope. 

163. The Telluric Lines. — There are many lines of the 
solar spectrum which vary considerably in intensity as the 
sun passes from the horizon to the meridian, being most 
intense when the sun is nearest the horizon, and when his 
rays are obliged to pass through the greatest depth of the 
earth's atmosphere. These lines are of atmospheric origin, 
and are due to the absorption of the aqueous vapor in our 
atmosphere. They are the same lines that are obtained 




when a candle or other artificial light is examined with a 
spectroscope through a long tube filled with steam. Since 
these lines are due to the absorption of our own atmos- 
phere, they are called telluric lines. A map of these lines 
is shown in Fig. 182. 

164. The Solar Lines. — After deducting the telluric 
lines, the remaining lines of the solar spectrum are of solar 
origin. They must be due to absorption which takes place 
in the sun's atmosphere. They are, in fact, the reversed 
spectra of the elements which exist in the solar atmosphere 
in the state of vapor : hence we conclude that the luminous 
surface of the sun is surrounded with an atmosphere of 
luminous vapors. The temperature of this atmosphere, at 



1 66 



ASTRONOMY. 



least near the surface of the sun, must be sufficient to 
enable all the elements known on the 
earth to exist in it as vapors. 

165. Chemical Constitution of the 
Sun's Atmosphere. — To find whether 
any element which exists on the earth 
is present in the solar atmosphere, we 
have merely to ascertain whether the 
bright lines of its gaseous spectrum 
are matched by dark lines in the 
solar spectrum when the two spectra 
are placed side by side. In Fig. 183, 
we have in No. 1 a portion of the 
red end of the solar spectra, and in 
No. 2 the spectrum of sodium vapor, 
both as obtained in the same spec- 
troscope by means of the compari- 
son prism. It will be seen that the 
double sodium line is exactly matched 
by a double dark line of the solar 
spectrum : hence we conclude that 
sodium vapor is present in the sun's 
atmosphere. Fig. 184 shows the 
matching of a great number of the 
bright lines of iron vapor by dark 
lines in the solar spectrum. This 
matching of the iron lines establishes 
the fact that iron vapor is present in 
the solar atmosphere. 

The following table (given by Pro- 
fessor Young) contains a list of all the 
elements which have, up to the present 
Fig. 182. t j me> b een detected with certainty in 

the sun's atmosphere. It also gives the number of bright lines 
in the spectrum of each element, and the number of those 



ASTRONOMY. 



167 



lines which have been matched by dark lines in the solar 
spectrum : — 



Elements. 


Bright Lines 
in Spectrum. 


Lines Reversed 
in Solar 
Spectrum. 


Observer. 


I. Iron 


600 


460 


Kirchhoff. 


2. Titanium . 






206 


Il8 


Thalen. 


3. Calcium . 






89 


75 


Kirchhoff. 


4. Manganese 






75 


57 


Angstrom. 


5. Nickel . . 






5 1 


33 


Kirchhoff. 


6. Cobalt . . 






86 


19 


Thalen. 


7. Chromium 






71 


18 


Kirchhoff. 


8. Barium . . 






26 


11 


Kirchhoff. 


9. Sodium 






9 


9 


Kirchhoff. 


10. Magnesium 






7 


7 


Kirchhoff. 


11. Copper? . 






15 


7? 


Kirchhoff. 


12. Hydrogen. 






5 


5 


Angstrom. 


13. Palladium. 






29 


5 


Lockyer. 


14. Vanadium. 






54 


4 


Lockyer. 


15. Molybdenum 






2 7 


4 


Lockyer. 


16. Strontium . 






74 


4 


Lockyer. 


17. Lead . . 






4i 


3 


Lockyer. 


18. Uranium . 






21 


3 


Lockyer. 


19. Aluminium 






14 


2 


Angstrom. 


20. Cerium. . 






64 


2 


Lockyer. 


21. Cadmium . 






20 


2 


Lockyer. 


2 ^ Oxygen a £ 
' Oxygen /3 > 






42 


12 ± bright 


H. Draper. 






4 


4? 


Schuster. 



In addition to the above elements, it is probable that several 
other elements are present in the sun's atmosphere ; since at 
least one of their bright lines has been found to coincide with 
dark lines of the solar spectrum. There are, however, a large 
number of elements, no traces of which have yet been detected ; 
and, in the cases of the elements whose presence in the solar 
atmosphere has been established, the matching of the lines is 
far from complete in the majority of the cases, as will be seen 
from the above table. This want of complete coincidence of 
the lines is undoubtedly due to the very high temperature of 



1 68 



ASTRONOMY. 



the solar atmosphere. We have already seen that the lines of 
the spectrum change with the temperature ; and, as the tem- 
perature of the sun is far higher than any that we can produce 
by artificial means, we might reasonably expect that it would 
cause the disappearance from the spectrum of many lines 
which we find to be present at our highest temperature. 

Lockyer maintains that the reason why no trace of the spec- 
tral lines of certain of our so-called elements is found in the 
solar atmosphere is, that these substances are not really elemen- 
tary, and that the intense heat of the sun resolves them into 
simpler constituents. 

Motion at the Surface of the Sun. 

i 66. Change of 'Pitch caused by Motion of Sounding 
Body. — When a sounding body is moving rapidly towards 



IS'9] 



W2 




us, the pitch of its note becomes somewhat higher than 
when the body is stationary; and, when such a body is 
moving rapidly from us, the pitch of its note is lowered 
somewhat. We have a good illustration of this change of 
pitch at a country railway station on the passage of an 
express-train. The pitch of the locomotive whistle "is con- 
siderably higher when the train is approaching the station 
than when it is leaving it. 

167. Explanation of the Change of Pitch produced by. 
Motion. — The pitch of sound depends upon the rapidity 
with which the pulsations of sound beat upon the drum of 



ASTRONOMY. 



169 



the ear. The more rapidly the pulsations follow each other, 

the higher is the pitch : hence 

the shorter the sound-waves 

(provided the sound is all the 

while travelling at the same 

rate)" the higher the pitch of 

the sound. Any thing, then, 

which tends to shorten the waves 

of sound tends also to raise its 

pitch, and any thing which tends 

to lengthen these waves tends to 

lower its pitch. 

When a sounding body is mov- 
ing rapidly forward, the sound- 
waves are crowded together a 
little, and therefore shortened; 
when it is moving backward, the 
sound-waves are drawn out, or 
lengthened a little. 

The effect of the motion of a 
sounding body upon the length 
of its sonorous waves will be 
readily seen from the following 
illustration : Suppose a number 
of persons stationed at equal 
intervals in a line on a long 
platform capable of moving back- 
ward and forward. Suppose the 
men are four feet apart, and all 
walking forward at the same 
rate, and that the platform is 
stationary, and that, as the men 
leave the platform, they keep on 
walking at the same rate : the men 
will evidently be four feet apart in the line in front of the 
platform, as well as on it. Suppose next, that the platform is 




I7O ASTRONOMY. 

moving forward at the rate of one foot in the interval between 
two men's leaving the platform, and that the men continue to 
walk as before : it is evident that the men will then be three 
feet apart in the line after they have left the platform. The 
forward motion of the platform has the effect of crowding the 
men together a little. Were the platform moving backward at 
the same rate, the men would be five feet apart after they had 
left the platform. The backward motion of the platform has 
the effect of separating the men from one another. 

The distance between the men in this illustration corre- 
sponds to the length of the sound-wave, or the distance between 
its two ends. Were a person to stand beside the line, and 
count the men that passed him in the three cases given above, 
he would find that more persons would pass him in the same 
time when the platform is moving forward than when it is 
stationary, and fewer persons would pass him in the same time 
when the platform is moving backward than when it is sta- 
tionary. In the same way, when a sounding body is moving 
rapidly forward, the sound-waves beat more rapidly upon the 
ear of a person who is standing still than when the body is at 
rest, and less rapidly when the sounding body is moving rapidly 
backward. 

Were the platform stationary, and were the person who is 
counting the men to be walking along the line, either towards 
or away from the platform, the effect upon the number of men 
passing him in a given time would be precisely the same as it 
would be were the person stationary, and the platform moving 
either towards or away from him at the same rate. So the 
change in the rapidity with which pulsations of sound beat upon 
the ear is precisely the same whether the ear is stationary and 
the sounding body moving, or the sounding body is stationary 
and the ear moving. 

1 68. Change of Refrangibility due to the Motion of a 
Luminous Body. — Refrangibility in light corresponds to 
pitch in sound, and depends upon the length of the lumi- 
nous waves. The shorter the luminous waves, the greater 
the refrangibility of the waves. Very rapid motion of a 
luminous body has the same effect upon the length of the 



ASTRONOMY. I7I 

luminous waves that motion of a sounding body has upon 
the length of the sonorous waves. When a luminous body 
is moving very rapidly towards us, its luminous waves are 
shortened a little, and its light becomes a little more 
refrangible ; when the luminous body is moving rapidly 
from us, its luminous waves are lengthened a little, and its 
light becomes a little less refrangible. 

169. Displacement of Spectral Lines. — In examining the 
spectra of the stars, we often find that certain of the dark 
lines are displaced somewhat, either towards the red or the 
violet end of the spectrum. As the dark lines are in the 
same position as the bright lines of the absorbing vapor 
would be, a displacement of 
the lines towards the red end 
of the spectrum indicates a 
lowering of the refrangibility of 
the rays, due to a motion of 
the luminous vapor away from 
us ; and a displacement of the 
lines towards the violet end of 
the spectrum indicates an in- 
crease of refrangibility, due to a 
motion of the luminous vapor 

towards us. From the amount of the displacement of the 
lines, it is possible to calculate the velocity at which the 
luminous gas is moving. In Fig. 185 is shown the dis- 
placement of the F line in the spectrum of Sirius. This 
is one of the hydrogen lines. R V is the spectrum, R 
being the red, and V the violet end. The long vertical 
line is the bright F line of hydrogen, and the short 
dark line to the left of it is the position of the F line 
in the spectrum of Sirius. It is seen that this line is dis- 
placed somewhat towards the red end of the spectrum. 
This indicates that Sirius must be moving from us ; and 
the amount °f tne displacement indicates that the star 




172 ASTRONOMY. 

must be moving at the rate of some twenty-five or thirty 
miles a second. 

170. Contortion of Lines on the Disk of the Sun. — 
Certain of the dark lines seen on the centre of the sun's 
disk often appear more or less distorted, as shown in 
Fig. 1 86, which represents the- contortion of the hydrogen 
line as seen at various times. 1 and 2 indicate a rapid 
motion of hydrogen away from us, or a down-rush at the 
sun ; 3 and 4 (in which the line at the centre is dark on 
one side, and bent towards the red end of the spectrum, 
and bright on the other side with a distortion towards the 
violet end of the spectrum) indicate a down-rush of cool 
hydrogen side by side with an up- rush of hot and bright 




Fig. 186. 

hydrogen ; 5 indicates local down-rushes associated with 
quiescent hydrogen. 

The contorted lines, which indicate a violently agitated 
state of the sun's atmosphere, appear in the midst of other 
lines which indicate a quiescent state. This is owing to 
the fact that the absorption which produces the dark lines 
takes place at various depths in the solar atmosphere. 
There may be violent commotion in the lower layers of the 
sun's atmosphere, and comparative quiet in the upper layers. 
In this case, the lines which are due to absorption in the 
lower layers would indicate this disturbance by their contor- 
tions ; while the lines produced by absorption in the upper 
layers would be free from contortion. 



ASTRONOMY. 



173 



It often happens, too, that the contortions are confined 
to one set of lines of an element, while other lines of the 
same element are entirely free from contortions. This is 
undoubtedly due to the fact that different layers of the 
solar atmosphere differ greatly in temperature ; so that the 
same element would give one set of lines at one depth, and 
another set at another depth : hence commotion in the 
solar atmosphere at any particular depth would be indi- 
cated by the contortion of those lines of the element only 
which are produced by the temperature at that particular 
depth. 

A remarkable case of contortion witnessed by Professor 
Young is shown in Fig. 187. Three successive appearances 
of the C line are 
shown. The second 
view was taken three 
minutes after the first, 
and the third five 
minutes after the sec- 
ond. The contortion 
in this case indicated 

a velocity ranging Fig. 187. 

from two hundred to three hundred miles a second. 

171. Contortion of Lines on the Sun's Limb. — When 
the spectroscope is directed to the centre of the sun's 
disk, the distortion of the lines indicates only vertical 
motion in the sun's atmosphere ; but, when the spectro- 
scope is directed to the limb of the sun, displacements of 
the lines indicate horizontal motions in the sun's atmos- 
phere. When a powerful spectroscope is directed to the 
margin of the sun's disk, so that the slit of the collimator 
tube shall be perpendicular to the sun's limb, one or more 
of the dark lines on the disk are seen to be prolonged by 
a bright line, as shown in Fig. 188. But this prolongation, 
instead of being straight and narrow, as shown in the figure, 




174 



ASTRONOMY. 



is often widened and distorted in various ways, as shown in 
Fig. 189. In the left-hand portion of the diagram, the line 
is deflected towards the red end of the spectrum; this 
indicates a violent wind on the sun's surface blowing away 

from us. In the right- 
hand portion of the dia- 
gram, the line is deflected 
towards the violet end of 
the spectrum ; this indi- 
cates a violent wind blow- 
ing towards us. In the 
middle portion of the 
figure, the line is seen to 
be bent both ways; this 
indicates a cyclone, on 
one side of which the 
wind would be blowing 




Fig. 1 



from us, and on the other side towards us. 

The distortions of the solar lines indicate that the wind 
at the surface of the sun often blows with a velocity of 
from one hundred to three hundred miles a second. The 




most violent wind known on the earth has a velocity of 
a hundred miles an hour. 



ASTRONOMY. 



175 



III. THE PHOTOSPHERE AND SUN SPOTS. 

The Photosphere. 

172. The Granulation of the Photosphere. — When the 
surface of the sun is examined with a good telescope under 
favorable atmospheric conditions, it is seen to be composed 




Fig. 190. 

of minute grains of intense brilliancy and of irregular form, 
Moating in a darker medium, and arranged in streaks and 
groups, as shown in Fig. 190. With a rather low power, 
the general effect of the surface is much like that of rough 
drawing-paper, or of curdled milk seen from a little dis- 
tance. With a high power and excellent atmospheric con- 
ditions, the grains are seen to be irregular, rounded masses, 



176 



ASTRONOMY. 



some hundreds of miles in diameter, sprinkled upon a less 
brilliant background, and appearing somewhat like snow- 
flakes sparsely scattered over a grayish cloth. Fig. 191 is 
a representation of these grains according to Secchi. 

With a very powerful telescope and the very best atmos- 
pheric conditions, the grains themselves are resolved into 
granules, or little luminous dots, not more than a hundrec 
miles or so in diameter, which, by their aggregation, make 
up the grains, just as they, in their turn, make up the coarser 
masses of the solar surface. Professor Langley estimates 
that these granules constitute about one-fifth of the sun's 

surface, while they 
emit at least three- 
fourths of its light. 

173. Shape of the 
Grains. — The grains 
differ considerably in 
shape at different 
times and on differ- 
ent parts of the sun's 
surface. Nasmyth, in 
1 86 1, described them 
Fig. 191. as willow-leaves ir 

shape, several thousand miles in length, but narrow and 
with pointed ends. He figured the surface of the sun as 
a sort of basket-work formed by the interweaving of such 
filaments. To others they have appeared to have the form 
of rice-grains. On portions of the sun's disk the elemen- 
tary structure is often composed of long, narrow, blunt- 
ended filaments, not so much like willow-leaves as like bits 
of straw lying roughly parallel to each other, — a thatch- 
straw formation, as it has been called. This is specially 
common in the immediate neighborhood of the spots. 

174. Nature of the Grams. — The grains are, undoubt- 
edly, incandescent clouds floating in the sun's atmosphere, 







ASTRONOMY. 1 77 

and composed of partially condensed metallic vapors, just 
as the clouds of our atmosphere are composed of partially 
condensed aqueous vapor. Rain on the sun is composed 
of white-hot drops of molten iron and other metals ; and 
these drops are often driven with the wind with a velocity 
of. over a hundred miles a second. 

As to the forms of the grains, Professor Young says, " If 
one were to speculate as to the explanation of the grains 
and thatch-straws, it might be that the grains are the upper 
ends of long filaments of luminous cloud, which, over most 
of the sun's surface, stand approximately vertical, but in 
the neighborhood of a spot are inclined so as to lie nearly 
horizontal. This is not certain, though : it may be that the 
cloud-masses over the more quiet portions of the solar sur- 
face are really, as they seem, nearly globular, while near the 
spots they are drawn out into filamentary forms by atmos- 
pheric currents." 

1 75 Faculce. — The faculce are irregular streaks of greater 
brightness than the general surface, looking much like the 
flecks of foam on the surface of a stream below a water- 
fall. They are sometimes from five to twenty thousand 
miles in length, covering areas immensely larger than a 
terrestrial continent. 

These faculae are elevated regions of the solar surface, 
ridges and crests of luminous matter, which rise above the 
general level of the sun's surface, and protrude through the 
denser portions of the solar atmosphere. When one of 
these passes over the edge of the sun's disk, it can be seen 
to project, like a little tooth. Any elevation on the sun to 
be perceptible at all must measure at least half a second 
of an arc, or two hundred and twenty-five miles. 

The faculae are most numerous in the neighborhood 
of the spots, and much more conspicuous near the limb 
of the sun than near the centre of the disk. Fig. 192 gives 
the general appearance of the faculae, and the darkening 



i 7 8 



ASTRONOMY. 



of the limb of the sun. Near the spots, the faculae often 
undergo very rapid change of form, while elsewhere on the 
disk they change rather slowly, sometimes undergoing little 
apparent alteration for several days. 

176. Why the Faculce are most Conspicuous near the 
Limb of the Sun. — The reason why the faculae are most 
conspicuous near the limb of the sun is this : The luminous 
surface of the sun is covered with an atmosphere, which, 
though not very thick compared with the diameter of the 
sun, is still sufficient to absorb a good deal of light. Light 





Fig. 192. 

coming from the centre of the sun's disk penetrates this 
atmosphere under the most favorable conditions, and is but 
slightly reduced in amount. The edges of the disk, on the 
other hand, are seen through a much greater thickness of 
atmosphere : and the light is reduced by absorption some 
seventy-five per cent. Suppose, now, a facula were suf- 
ficiently elevated to penetrate quite through this atmosphere. 
Its light would be undimmed by absorption on any part 
of the sun's disk ; but at the centre of the disk it would 
be seen against a background nearly as bright as itself, 
while at the margin it would be seen against one only a 






ASTRONOMY. 



179 



quarter as bright. It is evident that the light of any facula, 
owing to the elevation, would be reduced less rapidly as 
we approach the edge of the disk than that of the general 
surface of the sun, which lies at a lower level. 

Sun-Spots. 

177. General Appearance of Sun-Spots. — The general 
appearance of a well-formed sun-spot is shown in Fig. 193. 
The spot consists of a very dark central portion of irregu- 




lar shape, called the umbra, which is surrounded by a 
less dark fringe, called the penumbra. The penumbra is 
made up, for the most part, of filaments directed radially 
inward. 

There is great variety in the details of form in different 
sun-spots; but they are generally nearly circular during the 
middle period of their existence. During the period of 
their development and of their disappearance they are much 
more in egular in form. 



l8o ASTRONOMY. 

There is nothing like a gradual shading-off of the penum- 
bra, either towards the umbra on the one side, or towards 
the photosphere on the other. The penumbra is separated 
from both the umbra and the photosphere by a sharp line 
of demarcation. The umbra is much brighter on the inner 
than on the outer edge, and frequently the photosphere is 
excessively bright at the margin of the penumbra. The 
brightness of the inner penumbra seems to be due to the 
crowding together of the penumbral filaments where they 
overhang the edge of the umbra. 

There is a general antithesis between the irregularities of 
the outer and inner edges of the penumbra. Where an 
angle of the penumbral matter crowds in upon the umbra, 
it is generally matched by a corresponding outward exten- 
sion into the photosphere, and vice versa. 

The umbra of the spot is far from being uniformly dark. 
Many of the penumbral filaments terminate in little de- 
tached grains of luminous matter ; and there are also faint- 
er veils of a substance less brilliant, but sometimes rose- 
colored, which seem to float above the umbra. The umbra 
itself is made up of masses of clouds which are really 
intensely brilliant, and which appear dark only by contrast 
with the intenser brightness of the solar surface. Among 
these clouds are often seen one or more minute circular 
spots much darker than the rest of the umbra. These 
darker portions are called nuclei. They seem to be the 
mouths of tubular orifices penetrating to unknown depths. 
The faint veils mentioned above continually melt away, and 
are replaced by others in some different position. The 
bright granules at the tips of the penumbral filaments seem 
to sink and dissolve, while fresh portions break off to replace 
them. There is a continual indraught of luminous matter 
over the whole extent of the penumbra. 

At times, though very rarely, patches of intense brightness 
suddenly break out, remain visible for a few minutes, and 



ASTRONOMY. 151 

move over the spot with velocities as great as a hundred 
miles a second. 

The spots change their form and size quite percep- 
tibly from day to day, and sometimes even from hour to 
hour. 

178. Duration of Sim-Spots. — The average life of a 
sun-spot is two or three months : the longest on record is 
that of a spot observed in 1840 and 1841, which lasted 
eighteen months. There are cases, however, where the dis- 
appearance of a spot is very soon followed by the appear- 
ance of another at the same point • and sometimes this 
alternate disappearance and re-appearance is several times 
repeated. While some spots are thus long-lived, others 
endure only a day or two, and sometimes only a few 
hours. 

179. Groups of Spots. — The spots usually appear not 
singly, but in groups. A large spot is often followed by a 
train of smaller ones to the east of it, many of which are 
apt to be irregular in form and very imperfect in structure, 
sometimes with no umbra at all, often with a penumbra only 
on one side. In such cases, when any considerable change 
of form or structure shows itself in the principal spot, it 
seems to rush westward over the solar surface, leaving its 
attendants trailing behind. When a large spot divides into 
two or more, as often happens, the parts usually seem to 
repel each other, and fly apart with great velocity. 

180. Size of the Spots. — The spots are sometimes of 
enormous size. Groups have often been observed covering 
areas of more than a hundred thousand miles square, and 
single spots occasionally measure from forty to fifty thousand 
miles in diameter, the umbra being twenty-five or thirty 
thousand miles across. A spot, however, measuring thirty 
thousand miles over all, may be considered a large one. 
Such a spot can easily be seen without a telescope when 
the brightness of the sun's surface is reduced by clouds or 



182 



ASTRONOMY. 



nearness to the horizon, or by the use of colored glass. 
During the years 1871 and 1872 spots were visible to the 
naked eye for a considerable portion of the time. The 
largest spot yet recorded was observed in 1858. It had a 
breadth of more than a hundred and forty-three thousand 
miles, or nearly eighteen times the diameter of the earth, 
and covered about a thirty-sixth of the whole surface of 
the sun. 

Fig. 194 represents a group of sun-spots observed by 




Professor Langley, and drawn on the same scale as the small 
circle in the upper left-hand corner, which represents the 
surface of half of our globe. 

181. The Penumbral Filaments. — Not unfrequently the 
penumbral filaments are curved spirally, indicating a cyclonic 
action, as shown in Fig. 195. In such cases the whole spot 
usually turns slowly around, sometimes completing an entire 
revolution in a few days. More frequently, however, the 
spiral motion lasts but a short time ; and occasionally, after 
continuing for a while in one direction, the motion is 
reversed. Very often in large spots we observe opposite 






ASTRONOMY. 1 83 

spiral movements in different portions of the umbra, as 
shown in Figs. 196 and 197. 




Fig. 19s. 

Neighboring spots show no tendency to rotate in the 




Fig. 196. 

same direction. The number of spots in which a decided 
cyclonic motion (like that shown in Fig. 198) appears is 



1 84 



ASTRONOMY. 






comparatively small, not exceeding two or three per cent 
of the whole. 




Fig. 



Plate II. represents a typical sun-spot as delineated by 





a 




nOnn .^^^ 








RjPJT 




JRI ' m 








^mKmm ?Vm 




•'&■ Jflfi H»f ' *f 




J3 'HI 


■'c3Pv>-< 


m^^i^ysS^^^Ss^^^H Mfrfe 'j'z: ■.?$• 




^mi HKfedKf 


<5pf~ w 


" ^^«p^^S^^*"^i^^^l 




'—" ■* ^mNIsi 




sS#s^ ~"^-~ jBsiifir 




IP*' 



Fig. 198. 



Professor Langley. At the left-hand and upper portions of 
this great spot the filaments present the ordinary appearance, 






PLATE H. 




:v v ^ 




ASTRONOMY. 1 85 

while at the lower edge, and upon the great overhanging 
branch, they are arranged very differently. The feathery 
brush below the branch, closely resembling a frost-crystal 
on a window-pane, is as rare as it is curious, and has not 
been satisfactorily explained. 

182. Birth and Decay of Sim-Spots. — The formation of 
a spot is sometimes gradual, requiring days or even weeks 
for its full development ; and sometimes a single day suf- 
fices. Generally, for some time before its appearance, there 




is an evident disturbance of the solar surface, indicated espe- 
cially by the presence of many brilliant faculae, among which 
pores, or minute black dots, are scattered. These enlarge, 
and between them appear grayish patches, in which the 
photospheric structure is unusually evident, as if they were 
caused by a dark mass lying below a thin veil of luminous 
filaments. This veil seems to grow gradually thinner, and 
finally breaks open, giving us at last the complete spot with 
its penumbra. Some of the pores coalesce with the princi- 
pal spot, some disappear, and others form the attendant 



1 86 



ASTRONOMY. 



train before described (179). The spot whea once formec 
usually assumes a circular form, and remains without striking 
change until it disappears. As its end approaches, the 
surrounding photosphere seems to crowd in, and overwhelm 
the penumbra. Bridges of light (Fig. 199), often much 
brighter than the average of the solar surface, push across 
the umbra ; the arrangement of the penumbra filaments 
becomes confused ; and, as Secchi expresses it, the lumi- 
nous matter of the photosphere seems to tumble pell-mell 
into the chasm, which disappears, and leaves a disturbed 
surface marked with faculae, which, in their turn, gradually 
subside. 

183. Motion of Sun- Spots. — The spots have a regular 
motion across the disk of the sun 
from east to west, occupying about 
twelve days in the transit. A spot 
generally appears first on or near 
the east limb, and, after twelve or 
fourteen days, disappears at the west 
limb. At the end of another four- 
teen days, or more, it re-appears at 
the east limb, unless, in the mean 

time, it has vanished from sight entirely. This motion of 
the spots is indicated by the arrow in Fig. 200. The 
interval between two successive appearances of the same 
spot on the eastern edge of the sun is about twenty-seven 
days. 

184. The Rotation of the Sun. — The spots are evidently 
carried around by the rotation of the sun on its axis. It is 
evident, from Fig. 201, that the sun will need to make more 
than a complete rotation in order to bring a spot again 
upon the same part of the disk as seen from the earth. 
£ represents the sun, and E the earth. The arrows indicate 
the direction of the sun's rotation. When the earth is at E, 
a spot at a would be seen at the centre of the solar disk. ! 




ASTRONOMY. 



I87 



While the sun is turning on its axis, the earth moves in its 
orbit from E to E r : hence the sun must make a complete 
rotation., and turn from a to a' in addition, in order to 
bring the spot again to the centre of the disk. To carry 
the spot entirely around, and 
then on to a r , requires about 
twenty-seven days. From this 
sy nodical period of the spot, 
as it might be called, it has 
been calculated that the sun 
must rotate on its axis in 
about twenty- five days. 

185. The Inclination of the 
Sun 's Axis. — The paths de- 
scribed by sun-spots across 
the solar disk vary with the 
position of the earth in its 
orbit, as shown in Fig. 202. 
We therefore conclude that 
the sun's axis is not perpendicular to the plane of the 
earth's orbit. The sun rotates on its axis from west to east, 
and the axis leans about seven degrees from the perpendicu- 
lar to the earth's orbit. 





Fig. 202. 

186. The Proper Motion of the Spots. — When the 
period of the sun's rotation is deduced from the motion of 
spots in different solar latitudes, there is found to be con- 
siderable variation in the results obtained. Thus spots near 



188 



ASTRONOMY. 







Fig. 203. 



the equator indicate that the sun rotates in about twenty-five 
days; while those in latitude 20 indicate a period about 

eighteen hours 

longer; and those 
in latitude 30 a 
period of twenty- 
seven days and a 
half. Strictly speak- 
ing, the sun j as a 
whole, has no sin- 
gle period of rota- 
tion ; but different 
portions of its sur- 
face perform their 
revolutions in dif- 
ferent times. The 
equatorial regions 
not only move more rapidly in miles per hour than the 
rest of the solar surface, but they complete the entire rota- 
tion in shorter time. 

There appears to 
be a peculiar surface- 
drift in the equatorial 
regions of the sun, 
the cause of which 
is unknown, but which 
gives the spots a 
proper motion ; that 
is, a motion of their 
own, independent of 
the rotation of the 
sun. Fi s- 2 °4- 

187. Distribution of the Sun- Spots. — The sun-spots are 
not distributed uniformly over the sun's surface, but occur 
mainly in two zones on each side of the equator, and be- 




ASTRONOMY. 



189 



tween the latitudes of io° and 30 , as shown in Fig. 203. 
On and near the equator itself they are comparatively rare. 
There are still fewer beyond 35 ° of latitude, and only a 
single spot has ever been recorded more than 45 ° from the 
solar equator. 

Fig. 204 shows the distribution of 
the sun-spots observed by Carrington 
during a period of eight years. The 
irregular line on the left-hand side of 
the figure indicates by its height the 
comparative frequency with which the 
spots occurred in different latitudes. 
In Fig. 205 the same thing is indicated 
by different degrees of darkness in the 
shading of the belts. 

188. The Periodicity of the Spots. — 
Careful observations of the solar spots 
indicate a period of about eleven years 
in the spot-producing activity of the 
sun. During two or three years the 
spots increase in number and in size ; 
then they begin to diminish, and reach 
a minimum five or six years after the 
maximum. Another period of about 
six years brings the return of the maxi- 
mum. The intervals are, however, 
somewhat irregular. 

Fig. 206 gives a graphic representa- 
tion of the periodicity of the sun-spots. 
The height of the curve shows the 
frequency of the sun-spots in the years FI s- 2 °5- 

given at the bottom of the figure. It appears, from an 
examination of this sun-spot curve, that the average inter- 
val from a minimum to the next following maximum is 
only about four years and a half, while that from a maxi- 



190 



ASTRONOMY. 



mum to the next following minimum is six years and six- 
tenths. The disturbance which produces the sun-spots is 

developed suddenly, but dies away 

gradually. 



189. Connection between Sim- 
Spots and Terrestrial Magnetism. 
— The magnetic needle does not 
point steadily in the same direction, 
but is subject to various disturb- 
ances, some of which are regular, 
and others irregular. 

(1) One of the most noticeable 
of the regular magnetic changes is 
the so-called diurnal oscillation. 
During the early part of the day 
the north pole of the needle moves 
toward the west in our latitude, 
returning to its mean position about 
ten p.m., and remaining nearly 
stationary during the night. The 
extent of this oscillation in the 
United States is about fifteen min- 
utes of arc in summer, and not 
quite half as much in winter ; but 
it differs very much in different 
localities and at different times, and 
the average diurnal oscillation in 
any locality increases and decreases 
pretty regularly during a period of 
about eleven years. The maximum 
and minimum of this period of 
magnetic disturbance are found to 
coincide with the maximum and 
This is shown in Fig. 206, 



1 


HHSSi 
■Mil 

Hiss 

■■■■■ 
■■■■■ 


iMHiiig 

Mm 

HI 
HI 




■hSmhsim 

mm 

■■■■Kgnggigi 




IBK2JH IBffll 




jgjHBjjjjjBj 








!i!iiKi 




SSHSBl M«E£| 


■ipi iHeiiM 



Fig. 206. 

minimum of the sun-spot period. 

in which the dotted lines indicate the variations in the intensity 

of the magnetic disturbance. 

(2) Occasionally so-called magnetic storms occur, during 
which the compass-needle is sometimes violently disturbed. 



ASTRONOMY. 



I 9 I 



oscillating five degrees, or even ten degrees, within an hour 
or two. These storms are generally accompanied by an aurora, 
and an aurora is always accompanied by magnetic disturbance. 
A careful comparison of aurora observations with those of sun- 
spots shows an almost perfect parallelism between the curves 
of auroral and sun-spot frequency. 




Fig. 207. 

(3) A number of observations render it very probable that 
every intense disturbance of the* solar surface is propagated 
to our terrestrial magnetism with the speed of light. 

Fig. 207 shows certain of the solar lines as they were 
observed by Professor Young on Aug. 3, 1872. The contor- 
tions of the F line indicated an intense disturbance in the 




Fig. 208. 

atmosphere .of the sun. There were three especially notable 
paroxysms in this distortion, occurring at a quarter of nine, 
half-past ten, and ten minutes of twelve, a.m. 

Fig. 208 shows the curve of magnetic disturbance as traced 
at Greenwich on the same day. It will be seen from the curve 
that it was a day of general magnetic disturbance. At the 






I92 ASTRONOMY. 






times of the three paroxysms, which are given at the bottom 
of the figure, it will be observed that there is a peculiar shiver- 
ing of the magnetic curve. 

190. The Spots ai-e Depressions in the Photosphere. — 
This fact was first clearly brought out by Dr. Wilson of 
Glasgow, in 1769, from observations upon the penumbra of 
a spot in November of that year. He found, that when 
the spot appeared at the eastern limb, or edge of the sun, 
just moving into sight, the penumbra was well marked on 
the side of the spot nearest to the edge of the disk ; while 
on the other edge of the spot, towards the centre of the 

sun, there was no penumbra 
visible at all, and the umbra 
itself was almost hidden, as if 
behind a bank. When the spot 
had moved a day's journey 
toward the centre of the disk, 
the whole of the umbra came 
into sight, and the penumbra on 
the inner edge of the spot 
began to be visible as a narrow 
line. After the spot was well 
advanced upon the disk, the penumbra was of the same 
width all around the spot. When the spot approached the 
sun's western limb, the same phenomena were repeated, but 
in the inverse order. The penumbra on the inner edge of 
the spot narrowed much faster than that on the outer, dis- 
appeared entirely, and finally seemed to hide from sight 
much of the umbra nearly a whole day before the spot 
passed from view around the limb. This is precisely what 
would occur (as Fig. 209 clearly shows) if the spot were a 
saucer-shaped depression in the solar surface, the bottom 
of the saucer corresponding to the umbra, and the sloping 
sides to the penumbra. 




ASTRONOMY. 



!93 



191. Sun-Spot Spectrum. — When the image of a sun-spot 
is thrown upon the slit of the spectroscope, the spectrum is 
seen to be crossed longitudinally by a continuous dark band, 
showing an increased general absorption in the region of the 
sun-spot. Many of the spectral lines are greatly thickened, as 




Fig. 210. 

shown in Fig. 210. This thickening of the lines shows that 
the absorption is taking place at a greater depth. New lines 
and shadings often appear, which indicate, that, in the cooler 
nucleus of the spot, certain compound vapors exist, which are 




These lines and 



Fig. 211. 

dissociated elsewhere on the sun's surface, 
shadings are shown in Fig. 211. 

It often happens that certain of the spectral lines are re- 
versed in the spectrum of the spot, a thin bright line appear- 
ing over the centre of a thick dark one, as shown in Fig. 212. 
These reversals are due to very bright vapors floating over the 
spot. 



194 



ASTRONOMY. 



At times, also, the spectrum of a spot indicates violent 
motion in the overlying gases by distortion and displacement 
of the lines. This phenomenon occurs oftener at points near 

the outer edge of the penum- 
bra than over the centre of 
the spot ; but occasionally the 
whole neighborhood is vio- 
lently agitated. In such cases, 
lines in the spectrum side by 
side are often affected in en- 
tirely different ways, one being 
greatly displaced while its 




Fig. 212. 



neighbor is not disturbed in 



the least, showing that the 
vapors which produce the lines are at different levels in the 
solar atmosphere, and moving independently of each other. 

192. The Cause and Nature of Sun-Spots. — According to 
Professor Young, 
the arrangement 
and relations of 
the photospheric 
clouds in the 
neighborhood of 
a spot are such as 
are represented in 
Fig. 213. " Over 
the sun's surface 
generally, these 
clouds probably 
have the form of 
vertical columns, 
as at a a. Just 
outside the spot, 
the level of the 
photosphere is 
the most part, overtopped by eruptions of hydrogen and 
usually raised into faculae, as at bb. These faculae are, for 
metallic vapors, as indicated by the shaded clouds. . . . While 
the great clouds of hydrogen are found everywhere upon the 1 




Fig. 213. 



ASTRONOMY. I95 

sun, these spiky, vivid outbursts of metallic vapors seldom 
occur except just in the neighborhood of a spot, and then only 
during its season of rapid change. In the penumbra of the 
spot the photospheric filaments become more or less nearly 
horizontal, as at pp ; in the umbra at u it is quite uncertain 
what the true state of affairs may be. We have conjecturally 
represented the filaments there as vertical also, but depressed 
and carried down by a descending current. Of course, the 
cavity is filled by the gases which overlie the photosphere ; and 
it is easy to see, that, looked at from above, such a cavity 
and arrangement of the luminous filaments would present the 
appearances actually observed." 

Professor Young also suggests that the spots may be depres- 
sions in the photosphere caused ' ; by the diminution of upward 
Pressure from below, in consequence of eruptions in the neigh- 
borhood; the spots thus being, so to speak, sinks in the 
photosphere. Undoubtedly the photosphere is not a strictly 
continuous shell or crust ; but it is heavy as compared with the 
uncondensed vapors in which it lies, just as a rain-cloud in our 
terrestrial atmosphere is heavier than the air; and it is proba- 
bly continuous enough to have its upper level affected by any 
diminution of pressure below r . The gaseous mass below the 
photosphere supports its "weight and the weight of the products 
of condensation, which must always be descending in an incon- 
ceivable rain and snow of molten and crystallized material. 
To all intents and purposes, though nothing but a layer of 
clouds, the photosphere thus forms a constricting shell, and 
the gases beneath are imprisoned and compressed. Moreover, 
at a high temperature the viscosity of gases is vastly increased, 
so that quite probably the matter of the solar nucleus resem- 
bles pitch or tar in its consistency more than what we usually 
think of as a gas. Consequently, any sudden diminution of 
pressure would propagate itself slowly from the point where 
it occurred. Putting these things together, it would seem, that, 
whenever a free outlet is obtained through the photosphere at 
any point, thus decreasing the inward pressure, the result would 
be the sinking of a portion of the photosphere somewhere in 
the immediate neighborhood, to restore the equilibrium ; and, if 
the eruption were kept up for any length of time, the depres- 



I96 ASTRONOMY. 

sion in the photosphere would continue till the eruption ceased. 
This depression, filled with the overlying gases, would constitute 
a spot. Moreover, the line of fracture (if we may call it so) at 
the edges of the sink would be a region of weakness in the 
photosphere, so that we should expect a series of eruptions 
all around the spot. For a time the disturbance, therefore, 
would grow, and the spot would enlarge and deepen, until, in 
spite of the viscosity of the internal gases, the equilibrium of 
pressure was gradually restored beneath. So far as we know 
the spectroscopic and visual phenomena, none of them con- 
tradict this hypothesis. There is nothing in it, however, to 
account for the distribution of the spots in solar latitudes, nor 
for their periodicity.'' 

IV. THE CHROMOSPHERE AND PROMINENCES. 

193. The Sun's Outer Atmosphere. — What we see of 
the sun under ordinary circumstances is but a fraction of 
his total bulk. While by far the greater portion of the 
solar mass is included within the photosphere, the larger 
portion of his volume lies without, and constitutes a gaseous 
envelope whose diameter is at least double, and its bulk 
therefore sevenfold, that of the central globe. 

This outer envelope, though mainly gaseous, is not spheri- 
cal, but has an exceedingly irregular and variable outline. 
It seems to be made up, not of regular strata of different 
density, like our atmosphere, but rather of flames, beams, 
and streamers, as transient and unstable as those of the 
aurora borealis. It is divided into two portions by a 
boundary as definite, though not so regular, as that which 
separates them both from the photosphere. The outer and 
far more extensive portion, which in texture and rarity seems 
to resemble the tails of comets, is known as the coronal 
at7nosphere, since to it is chiefly due the corona, or glory, 
which surrounds the darkened sun during an eclipse. 

194. The Chromosphere. — At the base of the coronal 
atmosphere, and in contact with the photosphere, is what 



ASTRONOMY. 



197 



resembles a sheet of scarlet fire. It appears as if countless 
jets of heated gas were issuing through vents over the 
whole surface, clothing it with flame, which heaves and 
tosses like the blaze of a conflagration. This is the chro- 
mosphere, or color-sphere. It owes its vivid redness to the 
predominance of hydrogen in the flames. The average 
depth of the chromosphere is not far from ten or twelve 
seconds, or five thousand or six thousand miles. 

195. The Prominences. — Here and there masses of this 
hydrogen, mixed with other substances, rise far above the 
general level into the coronal regions, where they float like 
clouds, or are torn 
to pieces by conflict- 
ing currents. These 
cloud - masses are 
known as solar promi- 
nences, or protuber- 
ances. 

196. Magnitude and 
Distribution of the 
Prominences. — The 
prominences differ 
greatly in magnitude. 
Of the 2,767 observed 
by Secchi, 1,964 attained an altitude of eighteen thousand 
miles; 751, or nearly a fourth of the whole, reached a 
height of twenty-eight thousand miles ; several exceeded 
eighty-four thousand miles. In rare instances they reach 
elevations as great as a hundred thousand miles. A. few 
have been seen which exceeded a hundred and fifty thou- 
sand miles ; and Secchi has recorded one of three hundred 
thousand miles. 

The irregular lines on the right-hand side of Fig. 214 
show the proportion of the prominences observed by Secchi, 
that were seen in different parts of the sun's surface. The 




Fig. 214. 



198 



ASTRONOMY. 



outer line shows the distribution of the smaller prominences, 
and the inner dotted line that of the larger prominences. 
By comparing these lines with those 
on the opposite side of the circle, 
which show the distribution of the 
spots, it will be seen, that, while the 
spots are confined mainly to two 
belts, the prominences are seen in all 
latitudes. 

197. The Spectrum of the Chro- 
mosphere. — The spectrum of the 
chromosphere is comparatively simple. 
There are eleven lines only which are 
always present ; and six of these are 
lines of hydrogen, and the others, 
with a single exception, are of un- 
known elements. There are sixteen 
other lines which make their appear- 
ance very frequently. Among these 
latter are lines of sodium, -magnesium, 
and iron. 

Where some special disturbance is 
going on, the spectrum at the base of 
the chromosphere is very complicated, 
consisting of hundreds of bright lines. 
"The majority of the lines, however, 
are seen only occasionally, for a few 
minutes at a time, when the gases 
and vapors, which generally lie low 
(mainly in the interstices of the 
clouds which constitute the photo- 
sphere), and below its upper surface, 
are elevated for the time being by some eruptive action. 
For the most part, the lines which appear only at such 
times are simply reversals of the more prominent dark lines 




ASTRONOMY. 1 99 

of the ordinary solar spectrum. But the selection of the 
lines seems most capricious : one is taken, and another left, 
though belonging to the same element, of equal intensity, 
and close beside the first." Some of the main lines of the 
chromosphere and prominences are shown in Fig. 215. 

198. Method of Studying the Chromosphere and Promi- 
nences. — Until recently, the solar atmosphere could be seen 
only during a total eclipse of the sun ; but now the spectro- 
scope enables us to study the chromosphere and the promi- 
nences with nearly the same facility as the spots and faculas. 

The protuberances are ordinarily invisible, for the same 
reason that the stars cannot be seen in the daytime : they are 
hidden by the intense light reflected from our own atmosphere. 
If we could only get rid of this aerial illumination, without at 
the same time weakening the light of 
the prominences, the latter would be- 
come visible. This the spectroscope 
enables us to accomplish. Sin'ce the 
air-light is reflected sunshine, it of 
course presents the same spectrum as 
sunlight, — a continuous band of color 
crossed by dark lines. Now, this sort 
of spectrum is weakened by increase of dispersive power (159), 
because the light is spread out into a longer ribbon, and made 
to cover a greater area. On the other hand, the spectrum of 
the prominences, being composed of bright lines, undergoes 
no such diminution by increased dispersion. 

When the spectroscope is used as a means of examining the 
prominences, the slit is more or less widened. The telescope 
is directed so that the image of that portion of the solar limb 
which is to be examined shall be tangent to the opened slit, 
as in Fig. 216, which represents the slit-plate of the spectro- 
scope of its actual size, with the image of the sun in the 
proper position for observation. 

If, now, a prominence exists at this part of the solar limb, 
and if the spectroscope itself is so adjusted that the C line 
falls in -the centre of fhe field of view, then one will see some- 
thing like Fig. 217. "The red portion of the spectrum will 




200 



ASTRONOMY. 



stretch athwart the field of view like a scarlet ribbon with a 
darkish band across it; and in that band will appear the promi- 
nences, like scarlet clouds, so like our own terrestrial clouds, 
indeed, in form and texture, that the resemblance is quite 
startling. One might almost think he was looking out through 
a partly-opened door upon a sunset sky, except that there is 
no variety or contrast of color; all the cloudlets are of the 
same pure scarlet hue. Along the edge of the opening is seen 
the chromosphere, more brilliant than the clouds which rise 
from it or float above it, and, for the most part, made up of 
minute tongues and filaments." 




Fig. 217. 



199. Quiescent Prominences. — The prominences differ 

as widely in form 
and structure as in 
magnitude. The two 
principal classes are 
the quiescent, cloud- 
formed, or hydrogen- 
ous, and the eruptive, 
or metallic. 

The quiescent 

prominences resem- 
ble almost exactly 
our terrestrial clouds, and differ among themselves in the 
same manner. They are often of enormous dimensions, 
especially in horizontal extent, and are comparatively per- 
manent, often undergoing little change for hours and days. 
Near the poles they sometimes remain during a whole solar 
revolution of twenty-seven days. Sometimes they appear 
to lie upon the limb of the sun, like a bank of clouds in 
the terrestrial horizon, probably because they are so far 
from the edge that only their upper portions are in sight. 
When fully seen, they are usually connected to the chromo- 
sphere by slender columns, generally smallest at the base, 
and often apparently made up of separate filaments closely 



PLATE HL 




ASTRONOMY. 



20 1 



1 m )$j^L 




-- 



intertwined, and expanding upward. Sometimes the whole 
under surface is fringed with pendent filaments. Sometimes 
they float entirely 
free from the chro- 
mosphere ; and in 
most cases the 
larger clouds are 
attended by de- 
tached cloudlets. 
Various forms of 
quiescent promi- 
nences are shown 
in Plate Ill- 
Other forms are 
given in Figs. 218 
and 219. 

Their spectrum Fig. 218. 

is usually very simple, consisting of the four lines of hydro- 
gen and the orange 
D l : hence the ap- 
pellation hydroge- 
nous. Occasionally 
the sodium and 
magnesium lines 
also appear, even 
near the tops of 
the clouds. 

200. Eruptive 
Prominences. — The 
eruptive promi- 
nences ordinarily 
consist of brilliant 
FI s- 2I 9- spikes or jets, which 

change very rapidly in form and brightness. As a rule, their 
altitude is not more than twenty thousand or thirty thousand 




202 ASTRONOMY. 

miles ; but occasionally they rise far higher than even the 
largest of the quiescent protuberances. Their spectrum is 
very complicated, especially near their base, and often filled 
with bright lines. The most conspicuous lines are those of 
sodium, magnesium, barium, iron, and titanium : hence 
Secchi calls them metallic prominences. 

They usually appear in the immediate vicinity of a spot, 
never very near the solar poles. They change with such 
rapidity, that the motion can almost be seen with the eye. 




Fig. 220. 

Sometimes, in the course of fifteen or twenty minutes, a 
mass of these flames, fifty thousand miles high, will undergo 
a total transformation ; and in some instances their com- 
plete development or disappearance takes no longer time. 
Sometimes they consist of pointed rays, diverging in all 
directions, as represented in Fig. 220. " Sometimes they 
lock like flames, sometimes like sheaves of grain, some- 
times like whirling water-spouts capped with a great cloud ; 
occasionally they present most exactly the appearance of 
jets of liquid fire, rising and falling in graceful parabolas; 
frequently they carry on their edges spirals like the volutes 



ASTRONOMY. 



203 




of an Ionic column ; and continually they detach fila- 
ments, which rise to a great elevation, gradually expand- 
ing and growing fainter as they ascend, until the eye loses 
them." 

201. Change of 
Form in Prominences. 
— Fig. 221 represents 
a prominence as 
seen by Professor 
Young, Sept. 7, 18 71. 
It was an immense 
quiescent cloud, a 
hundred thousand miles long and fifty-four thousand miles 
high. At a there was a brilliant lump, somewhat in the 
form of a thunder-head. On returning to the spectro- 
scope less than half an hour 
afterwards, he found that the 
cloud had been literally blown 
into shreds by some incon- 
ceivable uprush from beneath. 
The prominence then pre- 
sented the form shown in 
Fig. 222. The debris of the 
cloud had already attained a 
height of a hundred thou- 
sand miles. While he was 
watching them for the next 
ten minutes, they rose, with 
a motion almost perceptible 
to the eye, till the upper- 
most reached an altitude of 
two hundred thousand miles. As the filaments rose, they 
gradually faded away like a dissolving cloud. 

Meanwhile the little thunder- head had grown and devel- 
oped into what appeared to be a mass of rolling and ever- 




204 



ASTRONOMY, 



changing flame. Figs. 223 and 224 give the appearance 





Fig. 223. 



Fig. 224. 



of this portion of the prominence at intervals of fifteen 
minutes. Other similar eruptions have been observed. 

V. THE CORONA. 

202. General Appearance of the Corona. — At the 
time of a total eclipse of the sun, if the sky is clear, the 
moon appears as a huge black ball, the illumination at 
the edge of the disk being just sufficient to bring out 
its rotundity. "From behind it," to borrow Professor 
Young's vivid description, " stream out on all sides radiant 
filaments, beams, and sheets of pearly light, which reach 
to a distance sometimes of several degrees from the solar 
surface, forming an irregular stellate halo, with the black 
globe of the moon in its apparent centre. The portion 
nearest the sun is of dazzling brightness, but still less bril- 
liant than the prominences which blaze through it like 
carbuncles. Generally this inner corona has a pretty uni- 
form height, forming a ring three or four minutes of arc 
in width, separated by a somewhat definite outline from 
the outer corona, which reaches to a much greater dis- 
tance, and is far more irregular in form. Usually there are 
several rifts, as they have been called, like narrow beams of 
darkness, extending from the very edge of the sun to the 
outer night, and much resembling the cloud-shadows which 
radiate from the sun before a thunder-shower; but the 
edges of these rifts are frequently curved, showing them 



ASTRONOMY. 



205 



to be something else than real shadows. Sometimes there 
are narrow bright streamers, as long as the rifts, or longer. 
These are often inclined, occasionally are even nearly 
tangential to the solar surface, and frequently are curved. 
On the whole, the corona is usually less extensive and 
brilliant over the solar poles, and there is a recognizable 




Fig. 225. 

tendency to accumulations above the middle latitudes, or 
spot-zones ; so that, speaking roughly, the corona shows a 
disposition to assume the form of a quadrilateral or four- 
rayed star, though in almost every individual case this form 
is greatly modified by abnormal streamers at some point or 
other." 

203. The Corona as seen at Recent Eclipses. — The 



206 



ASTRONOMY, 



corona can be seen only at the time of a total eclipse of 
the sun, and then for only a few minutes. Its form varies 
considerably from one eclipse to another, and apparently 
also during the same eclipse. At least, different observers 
at different stations depict the same corona under very 
different forms. Fig. 225 represents the corona of 1857 as 




Fig. 226. 

observed by Liais. In this view the petal-like forms, which 
have been noticed in the corona at other times, are espe- 
cially prominent. 

Fig. 226 shows the corona of i860 as it was observed 
by Temple, 

Fig, 227 shows the corona of 1867. This is interesting 
as being a corona at the time of sun-spot minimum, 



ASTRONOMY. 



207 



Fig. 228 represents the corona of 1868. This is a larger 
and more irregular corona than usual. 

The corona of 1869 is shown in Fig. 229. 

Fig. 230 is a view of the corona of 1871 as seen by 
Capt. Tupman. 

Fig. 231 shows the same corona as seen by Fcenander. 




Fig. 227. 

Fig. 232 shows the same corona as photographed by 
Davis. 

Fig- 2 33 shows the corona of 1878 made up from several 
views as combined by Professor Young. 

204. The Spectrum of the Corona. — The chief line in the 
spectrum of the corona is the one usually designated as 1474, 
and now known as the coronal line. It is seen as a dark line 



208 



ASTRONOMY. 



on the disk of the sun ; and a spectroscope of great dispersive 
power shows this dark line to be closely double, the lower 
component being one of the iron lines, and the upper the 
coronal line. This dark line is shown at x, Fig. 234. 

Besides this bright line, the hydrogen lines appear faintly 
in the spectrum of the corona. The 1474 line has been 




Fig. 228. 

sometimes traced with the spectroscope to an elevation of 
nearly twenty minutes above the moon's limb, and the hydro- 
gen lines nearly as far; and the lines were just as strong 
in the middle of a dark rift as anywhere else. 

The substance which produces the 1474 line is unknown 
as yet. It seems to be something with a vapor-density far 
below that of hydrogen, which is the lightest substance of 
which we have any knowledge. I can hardly be an "alio- 



ASTRONOMY. 



209 



tropic " form of any terrestrial element, as some scientists have 
suggested: for in the most violent disturbances in prominences 
and near sun-spots, when the lines of hydrogen, magnesium, 
and other metals, are contorted and shattered by the rush of 
the contending elements, this line alone remains fine, sharp, 
and straight, a little brightened, but not otherwise affected. 




Fig. 22Q. 



For the present it remains, like a few other lines in the spec- 
trum, an unexplained mystery. 

Besides bright lines, the corona shows also a faint continu- 
ous spectrum, in which have been observed a few of the more 
prominent dark lines of the solar spectrum. 

This shows, that, while the corona may be in the main 
composed of glowing gas (as indicated by the bright lines 
of its spectrum), it also contains considerable matter in such 



2IO 



ASTRONOMY. 



a state as to reflect the sunlight, probably in the form of dust 
or fog. 

V. ECLIPSES. 

205. The Shadows of the Earth and Moon. — The 
shadows cast by the earth and moon are shown in Fig. 235. 




Fig. 230. 



Each shadow is seen to be made up of a dark portion 
called the umbra, and of a lighter portion called the 
penitmbra. The light of the sun is completely excluded 
from the umbra, but only partially from the penumbra. 
The umbra is in the form of a cone, with its apex away 
from the sun ; though in the case of the earth's shadow 
it tapers very slowly. The penumbra surrounds the umbra, 



ASTRONOMY. 



21 I 



and increases in size as we recede from the sun. The axis 
of the earth's shadow lies in the plane of the ecliptic, which 
in the figure is the surface of the page. As the moon's 
orbit is inclined five degrees to the plane of the ecliptic, 
the axis of the moon's shadow will sometimes lie above, 




Fig. 231. 

ind sometimes below, the ecliptic. It will lie on the ecliptic 
mly when the moon is at one of her nodes. 

206. When there will be an Eclipse of the Moon. — 
The moon is eclipsed whenever it passes into the umbra 
/ the earth's shadow. It will be seen from the figure that 
he moon can pass into the shadow of the earth only when 
he is in opposition, or at full. Owing to the inclina- 
ion of the moon's orbit to the ecliptic, the moon will pass 



212 



ASTRONOMY. 



either above or below the earth's shadow when she is at 
full, unless she happens to be near her node at this time : 
hence there is not an eclipse of the moon every month. 

When the moon simply passes into the penumbra of the 
earth's shadow, the light of the moon is somewhat dimmed, 




Fig. 232. 

but not sufficiently to attract attention, or to be denomi- 
nated an eclipse. 

207. The Lunar Ecliptic Limits. — In Fig. 236 the line! 
A B represents the plane of the ecliptic, and the line CD th< 
moon's orbit. The large black circles on the line A B repre 
sent sections of the umbra of the earth's shadow, and th< 
smaller circles on CD represent the moon at full. It will b( 
seen, that, if the moon is full at E, she will just graze the 



ASTRONOMY. 



213 



umbra of the earth's shadow. In this case she will suffer no 
eclipse. Were the moon full at any point nearer her node, as 
at F, she would pass into the umbra of the earth's shadow, and 
would be partially eclipsed. Were the moon full at c7, she 
would pass through the centre of the earth's shadow, and be 
totally eclipsed. 

It will be seen from the figure that full moon must occur 




Fig. 233. 

when the moon is within a certain distance from her node, in 
order that there may be a lunar eclipse ; and this space is 
called the lunar ecliptic limits. 

The farther the earth is from the sun, the less rapidly does 
its shadow taper, and therefore the greater its diameter at the 
distance of the moon ; and, the nearer the moon to the earth, 
the greater the diameter of the earth's shadow at the distance 
of the moon. Of course, the greater the diameter of the 



214 



ASTRONOMY. 






earth's shadow, the greater the ecliptic limits: hence the lunar 
ecliptic limits vary somewhat from time to time, according to 
the distance from the earth to the sun and from the earth 
to the moon. The limits within which an eclipse is inevitable 
under all circumstances are called the 77iinor ecliptic limits ;\ 
and those within which an eclipse is possible under some cir- 
cumstances, the major ecliptic limits. 

208. Lunar Eclipses. — Fig. 237 shows the path of the 
moon through the earth's shadow in the case of a partial 
eclipse. The magnitude of such an eclipse depends upon 
the nearness of the moon to her nodes. The magnitude of 
an eclipse is usually denoted in digits, a digit being one- 
twelfth of the diameter of the moon. 

Fig. 238 shows the path of the moon through the earth's 

shadow in the 
case of a total 
eclipse. It will 
be seen from 
the figure that 
it is not neces- 
sary for the moon 
to pass through 
Fig - 234 - the centre of the 

earth's shadow in order to 'have a total eclipse. When 
the moon passes through the centre of the earth's shadow, 
the eclipse is both total and central. 

At the time of a total eclipse, the moon is not entirely 
invisible, but shines with a faint copper-colored light. This 
light is refracted into the shadow by the earth's atmosphere, 
and its amount varies with the quantity of clouds and vapor 
in that portion of the atmosphere which the sunlight must 
graze in order to reach the moon. 

The duration of an eclipse varies between very wide 
limits, being, of course, greatest when the eclipse is central. 
A total eclipse of the moon may last nearly two hours, or, 




ASTRONOMY, 



215 



including the partial portions of the eclipse, three or four 
hours. 




Fig. 235. 

Every eclipse of the moon, whether total or partial, is 



2l6 



ASTRONOMY. 



visible at the same time to the whole hemisphere of the 
earth which is turned towards the moon ; and the eclipse 
will have exactly the same magnitude at every point of 
observation. 




Fig. 236. 



209. When there will be an Eclipse of the Sun. — 
There will be an eclipse of the sun ivheriever any portion 
of the moon's shadow is thrown on the earth. It will be 
seen from Fig. 235 that this can occur only when the moon 



JB Wiifik 


Ecliptic. pH 


■ - SaB Ecliptic 




:r 'W\ ^^^ 


°^!L--^\^i8 





Fig. 237. 

is in conjunction, or at new. It does not occur every 
month, because, owing to the inclination of the moon's orbit 
to the ecliptic, the moon's shadow is usually thrown either 
above or below the earth at the time of new moon. There 



ASTRONOMY. 21? 

can be an eclipse of the sun only when new moon occurs 
at or near one of the nodes of her orbit. 

210. Solar Ecliptic Limits. — The distances from the moon's 
node within which a new moon would throw some portion of 
its shadow on the earth so as to produce an eclipse of the 
sun are called the sola?- ecliptic limits. As in the case of 
the moon, there are major and minor ecliptic limits ; the former 
being the limits within which an eclipse of the sun is possi- 
ble under some circumstances, and the latter those under which 
an eclipse is inevitable under all circumstances. 







^^SSSi 


SH 


iifev 








fmk 


B^hS8»$m3** 














Sh 1BE«E£to 








\*^~- 


Ecliptic, 






/ Ecliptic. 


0**°^ 








Ipp 







Fig. 238. 

The limits within which a solar eclipse may occur are 
greater than those within which a lunar eclipse may occur. 
This will be evident from an examination of Fig. 235. Were 
the moon in that figure just outside of the lines A B and CD, 
it will be seen that the penumbra of her shadow would just 
graze the earth : hence the moon must be somewhere within 
the space bounded by these lines in order to cause an eclipse 
of the sun. Now, these lines mark the prolongation to the 
sun of the cone of the umbra of the earth's shadow: hence, 
in order to produce an eclipse of the sun, new moon must 
occur somewhere within this prolongation of the umbra of the 
earth's shadow. Now, it is evident that the diameter of this 



218 



ASTRONOMY. 



cone is greater on the side of the earth toward the sun than 
on the opposite side : hence the solar ecliptic limits are greater 
than the lunar ecliptic limits. 

211. Solar Eclipses. — An observer in the umbra of the 
moon's shadow would see a total eclipse of the sun, while 




Fig. 239. 

one in the penumbra would see only a partial eclipse. The 
magnitude of this partial eclipse would depend upon the 
distance of the observer from the umbra of the moon's 
shadow. 
The umbra of the moon's shadow is just about long 




Fig. 240. 

enough to reach the earth. Sometimes the point of this 
shadow falls short of the earth's surface, as shown in 
Fig. 239, and sometimes it falls upon the earth, as shown 
in Fig. 240, according to the varying distance of the sun 
and moon from the earth. The diameter of the umbra at 
the surface of the earth is seldom more than a hundred 



ASTRONOMY. 



219 



miles : hence the belt of a total eclipse is, on the average, 
not more than a hundred miles wide ; and a total eclipse 
seldom lasts more than five or six minutes, and sometimes 
only a few seconds. Owing, however, to the rotation of 
the earth, the umbra of the moon's shadow may pass over 
a long reach of the earth's surface. Fig. 241 shows the 




Fig. 241. 

track of the umbra of the moon's shadow over the earth 
in the total eclipse of i860. 

Fig. 242 shows the track of the total eclipse of 187 1 
across India and the adjacent seas. 

In a partial eclipse of the sun, more or less of one side 
of the sun's disk is usually concealed, as shown in Fig. 243. 
Occasionally, however, the centre of the sun's disk is cov- 
ered, leaving a bright ring around the margin, as shown in 
Fig. 244. Such an eclipse is called an annular eclipse. 



220 



ASTRONOMY, 



An eclipse can be annular only when the cone of the 
moon's shadow is too short to reach the earth, and then 




Fig. 242. 



only to observers who are in the central portion of the 
penumbra. 

212. Comparative Frequency of Solar and Lunar 
Eclipses. — There are more eclipses of the sun in the year 



ASTRONOMY. 



221 



than of the moon ; and yet, at any one place, eclipses of 
the moon are more frequent than those of the sun. 

There are more lunar than solar eclipses, because, as we 
have seen, the limits within which a solar eclipse may occur 
are greater than those within which a lunar eclipse may occur. 
There are more eclipses of the moon visible at any one place 
than of the sun ; because, as we have seen, an eclipse of the 





Fig. 244. 



Fig. 243. 

moon, whenever it does occur, is visible to a whole hemisphere 
at a time, while an eclipse of the sun is visible to only a por- 
tion of a hemisphere, and a total eclipse to only a very small 
portion of a hemisphere. A total eclipse of the sun is, there- 
fore, a very rare occurrence at any one place. 

The greatest number of eclipses that can occur in a year is 
seven, and the least number, two. In the former case, five 
may be of the sun and two of the moon, or four of the sun 
and three of the moon. In the latter case, both must be of the 



VI. THE THREE GROUPS OF PLANETS. 

I. GENERAL CHARACTERISTICS OF THE 
GROUPS. 

213. The Inner Group. — The inner group of planets 
is composed of Mercury, Venics, the Earth, and Mars : 
that is, of all the planets which lie between the asteroids 



222 



ASTRONOMY. 



and the sun. The planets of this group are comparatively 
small and dense. So far as known, they rotate on their 
axes in about twenty-four hours, and they are either entirely 
without moons, or are attended by comparatively few. 
The comparative sizes and eccentricities of the orbits of 




this group are shown in Fig. 245. The dots round the orbits 
show the position of the planets at intervals of ten days. 

214. The Outer Group. — The outer group of planets 
is composed of Jupiter, Saturn, Uranus, and Neptune. 
These planets are all very large and of slight density. So 
far as known, they rotate on their axes in about ten hours, 



ASTRONOMY. 



223 



and are accompanied with complicated systems of moons. 
Fig. 246, which represents the comparative sizes of the 
planets, shows at a glance the immense difference between 
those of the inner and outer group. Fig. 247 shows the 
comparative sizes and eccentricities of the orbits of the 
outer planets. The dots round the orbits show the position 
of the planets at intervals 
of a thousand days. 

215. The Asteroids. — 
Between the inner and 
outer groups of planets 
there is a great number 
of very small planets 
known as the minor plan- 
ets, or asteroids. Over 
two hundred planets be- 
longing to this group 
have already been dis- 
covered. Their orbits are 
shown by the dotted lines 
in Fig. 247. The sizes 
of the four largest of 
these planets, compared 
with the earth, are shown 
in Fig. 248. 

The asteroids of this 
group are distinguished from the other planets, not only by 
their small size, but by the great eccentricities and inclina- 
tions of their orbits. If we except Mercury, none of the 
larger planets has an eccentricity amounting to one-tenth 
the diameter of its orbit (43), nor is any orbit inclined more 
than two or three degrees to the ecliptic ; but the inclina- 
tions of many of the minor planets exceed ten degrees, and 
the eccentricities frequently amount to an eighth of the 
orbital diameter. The orbit of Pallas is inclined thirty-four 




• • 



Fig. 246. 



224 



ASTRONOMY, 






degrees to the ecliptic, while there are some planets of 
this group whose orbits nearly coincide with the plane of 
the ecliptic. 

Fig. 249 shows one of the most and one of the least 




eccentric of the orbits of this group as compared with that 
of the earth. 

The intricate complexity of the orbits of the asteroids is 
shown in Fig. 250. 



ASTRONOMY. 



225 



II. THE INNER GROUP OF PLANETS. 



Mercury. 




Fig. 248. 



216. The Orbit of Mercury.— The, orbit of Mercury is 
more eccentric than that of any of the larger planets, and 
it has also a greater 
inclination to the eclip- 
tic. Its eccentricity (43) 
is a little over a fifth, 
and its inclination to 
the ecliptic somewhat 
over seven degrees. The 
mean distance of Mer- 
cury from the sun is 
about thirty-five million miles ; but, owing to the great 
eccentricity of its orbit, its distance from the sun varies 

from about forty- 
three million miles 
at aphelion to 
about twenty-eight 
million at perihe- 
lion. 

217. Distance of 
Mercury from the 
Earth. — It is evi- 
dent, from Fig. 
251, that an infe- 
rior planet, like 
Mercury, is the 
whole diameter of 
its orbit nearer the 
earth at inferior conjunction than at superior conjunction : 
hence Mercury's distance from the earth varies considerably. 
Owing to the great eccentricity of its orbit, its distance 




Freia 

Fig. 249. 



226 



ASTRONOMY. 



from the earth at inferior conjunction also varies considera- 
bly. Mercury is nearest to the earth when its inferior 
conjunction occurs at its own aphelion and at the earth's 
perihelion. 

218. Apparent Size of Mercury. — Since Mercury's dis- 
tance from the earth is variable, the apparent size of the 




Fig. 250. 

planet is also variable. Fig. 252 shows its apparent size at 
its extreme and mean distances from the earth. Its appar- 
ent diameter varies from five seconds to twelve seconds. 

219. Volume and Density of Mercury. — The real 
diameter of Mercury is about three thousand miles. Its 
size, compared with that of the earth, is shown in Fig. 
253. The earth is about sixteen times as large as Mer- 



ASTRONOMY. 



227 



cury ; but Mercury is about one-fifth more dense than the 
earth. 

220. Greatest Elongation of Mercury. — Mercury, being 
an inferior planet (or one within the orbit of the earth), 
appears to oscillate 
to and fro across 
the sun. Its great- 
est apparent distance 
from the sun, or its 
greatest elongation, 
varies considerably. Ai 
The farther Mercury 
is from the sun, and 
the nearer the earth 
is to Mercury, the 
greater is its angular 
distance from the sun 
at the time of its 
greatest elongation. Under the most favorable circum- 
stances, the greatest elongation amounts to about twenty- 
eight degrees, and under the least favorable to only sixteen 
or seventeen degrees. 

221. Sidereal and Synodical Periods of Mercury. — Mer- 
cury accomplishes 
a complete revolu- 
tion around the sun 
in about eighty- 
eight days ; but it 
takes it a hundred 

Fig- 252. and sixteen days to 

pass from its greatest elongation east to the same elonga- 
tion again. The orbital motion of this planet is at the 
rate of nearly thirty miles a second. 

In Fig. 251, P'" represents elongation east of the sun, 
and P r elongation west. It will be seen that it is much 





228 



ASTRONOMY. 



farther from P' around to P'" than from P'" on to P' . 
Mercury is only about forty-eight days in passing from 
greatest elongation east to greatest elongation west, while 
it is about sixty-eight days in passing back again. 

222. Visibility of Mercury. — Mercury is too close to 




Fig. 253. 

the sun for favorable observation. It is never seen long 
after sunset, or long before sunrise, and never far from the 
horizon. When visible at all, it must be sought for low 
down in the west shortly after sunset, or low in the east 

shortly before sunrise, accord- 
ing as the planet is at its east 
or west elongation. It is often 
visible to the naked eye in our 
latitude \ but the illumination 
of the twilight sky, and the 
excess of vapor in our atmos- 
phere near the horizon, com- 
bine to make the telescopic 
study of the planet difficult 
Flg - 254 * and unsatisfactory. 

223, The Atmosphere and Surface of Mercury. — Mer- 
cury seems to be surrounded B by a dense atmosphere. One 
proof of the existence of such an atmosphere is furnished 
at the time of the planet's transit across the disk of the 
sun, which occasionally happens. The planet is then seen 









M&i 




Willis 


fc^"*:'^ 




1 "^ 


I" 













ASTRONOMY. 



229 



surrounded by a border, as shown in Fig. 254. A bright 
spot has also been observed on the dark disk of the planet 
during a transit, as shown in Fig. 255. The border around 
the planet seems to 
be due to the action 
of the planet's atmos- 
phere; but it is dif- 
ficult to account for 
the bright spot. 

Schroter, a cele- 
brated German as- 
tronomer, at about the 
beginning of the pres- 
ent century, thought 
that he detected spots 
and shadings on the 
disk of the planet, 
which indicated both 
the presence of an atmosphere and of elevations. The 
shading along the terminator, which seemed to indicate 
the presence of a twilight, and therefore of an atmos- 
phere, are shown in Fig. 256. It also shows the blunted 





Fig. 256. 

aspect of one of the cusps, which Schroter noticed at times, 
and which he attributed to the shadow of a mountain, 
estimated to be ten or twelve miles high. Fig. 257 shows 



230 



ASTRONOMY. 



this mountain near the upper cusp, as Schroter believed he 
saw it in the year 1800. By watching certain marks upon 
the disk of Mercury, Schroter came to the conclusion that 
the planet rotates on its axis in about twenty-four hours. 
Modern observers, with more powerful telescopes, have 
failed to verify Schroter's observations as to the indications 
of an atmosphere and of elevations. Nothing is known 
with certainty about the rotation of the planet. 

The border around Mercury, and the bright spot on its 
disk at the time of the transit of the planet across the sun, 
have been seen since Schroter's time, and the existence of 
these phenomena is now well established ; but astronomers 

are far from being agreed as to 
their cause. 

224. Intra- Mercurial Planets, 
— It has for some time been 
thought probable that there is a 
group of small planets between 
Mercury and the sun ; and at vari- 
ous times the discovery of such 
bodies has been announced. In 
1859 a French observer believed 
Fig. 257. that he had detected an intra- 

Mercurial planet, to which the name of Vulcan was given, 
and for which careful search has since been made, but with- 
out success. During the total eclipse of 1878 Professor 
Watson observed two objects near the sun, which he thought 
to be planets ; but this is still matter of controversy. 

Venus. 

225. The Orbit of Venus. — The orbit of Venus has 
but slight eccentricity, differing less from a circle than that 
of any other large planet. It is inclined to the ecliptic some- 1 
what more than three degrees. The mean distance of the 
planet from the sun is about sixty-seven million miles. 




ASTRONOMY. 23 I 

226. Distance of Venus from the Earth, — The dis- 
tance of Venus from the earth varies within much wider 
limits than that of Mercury. When Venus is at inferior 
conjunction, her distance from the earth is ninety- two 
million miles minus sixty-seven million miles, or twenty- 
five million miles ; and when at superior conjunction it is 
ninety-two million miles plus sixty-seven million miles, or 
a hundred and fifty-nine million miles. Venus is consid- 
erably more than six times as far off at superior conjunc- 
tion as at inferior conjunction. 

227. Apparent Size of Venus. — Owing to the great 




Fig. 258. 

variation in the distance of Venus from the earth, her 
apparent diameter varies from about ten seconds to about 
sixty-six seconds. Fig. 258 shows the apparent size of 
Venus at her extreme and mean distances from the earth. 

228. Volume and Density of Venus. — The real size of 
Venus is about the same as that of the earth, her diameter 
being only about three hundred miles less. The compara- 
tive sizes of the two planets are shown in Fig. 259. The 
density of Venus is a little less than that of the earth. 

229. The Greatest Elongation of Venus. — Venus, like 
Mercury, appears to oscillate to and fro across the sun. 
The angular value of the greatest elongation of Venus 
varies but slightly, its greatest value being about forty- seven 
degrees. 



232 ASTRONOMY. 






230. Sidereal and Sy nodical Periods of Venus. — The 
sidereal period of Venus, or that of a complete revolution 
around the sun, is about two hundred and twenty-five days ; 
her orbital motion being at the rate of nearly twenty-two 
miles a second. Her sy nodical period, or the time it takes 
her to pass around from her greatest eastern elongation 
to the same elongation again, is about five hundred and 
eighty-four days, or eighteen months. Venus is a hun- 
dred and forty-six days, or nearly five months, in passing 
from her greatest elongation east through inferior conjunc- 
tion to her greatest elongation west. 







Fig. 259. 

231. Venus as a Morning and an Evening Star. — For 
a period of about nine months, while Venus is passing from 
superior conjunction to her greatest eastern elongation, she 
will be east of the sun, and will therefore set after the 
sun. During this period she is the evening star, the Hespe- 
rus of the ancients. While passing from inferior conjunc- 
tion to superior conjunction, Venus is west of the sun, and 
therefore rises before the sun. During this period of nine 
months she is the morni?ig star, the Phosphorus, or Lucifer, 
of the ancients. 

232. Brilliancy of Venus. — Next to the sun and moon, 
Venus is at times the most brilliant object in the heavens, 
being bright enough to be seen in daylight, and to cast 






ASTRONOMY. 233 

a distinct shadow at night. Her brightness, however, varies 
considerably, owing to her phases and to her varying dis- 
tance from the earth. She does not appear brightest when 
at full, for she is then farthest from the earth, at superior 
conjunction : nor does she appear brightest when nearest the 
earth, at inferior conjunction, for her phase is then a thin 
crescent (see Fig. 258). She is most conspicuous while 
passing from her greatest eastern elongation to her great- 
est western elongation. After she has passed her eastern 
elongation, she becomes brighter and brighter till she is 
within about forty degrees of the sun. Her phase at 
this point in her orbit is 
shown in Fig. 260. Her 
brilliancy then begins to 
wane, until she comes too 
near the sun to be visi- 
ble. When she re-appears 
on the west of the sun. 
she again becomes more 
brilliant ; and her brilliancy 
increases till she is about 
forty degrees from the sun, 
when she is again at her 

1 • 1 t r Fic 260 

brightest. \ enus passes 

from her greatest brilliancy as an evening star to her great- 
est brilliancy as a morning star in about seventy-three days. 
She has the same phase, and is at the same distance from 
the earth, in both cases of maximum brilliancy. Of course, 
the brilliancy of Venus when at the maximum varies some- 
what from time to time, owing to the eccentricities of the 
orbits of the earth and of Venus, which cause her distance 
irom the earth, at her phase of greatest brilliancy, to vary. 
She is most brilliant when the phase of her greatest bril- 
liancy occurs when she is at her aphelion and the earth at 
its perihelion. 




234 



ASTRONOMY. 



233. The Atmosphere and Surface of Venus. — Schroter 
believed that he saw shadings and markings on Venus simi- 
lar to those on Mercury, indicating the presence of an 
atmosphere and of elevations on the surface of the planet. 
Fig. 261 represents the surface of Venus as it appeared 




Fig. 261. 

to this astronomer. By watching certain markings on the 
disk of Venus, Schroter came to the conclusion that Venus 
rotates on her axis in about twenty-four hours. 

It is now generally conceded that Venus has a dense 
atmosphere ; but Schroter's obser- 
vations of the spots on her disk 
have not been verified by modern 
astronomers, and we really know 
nothing certainly of her rotation. 

234. Transits of Venus. — When 
Venus happens to be near one of 
the nodes of her orbit when she is ] 
in inferior conjunction, she makes, 
a transit across the sun's disk. 
These transits occur in pairs, separated by an interval of 
over a hundred years. The two transits of each pair are 
separated by an interval of eight years, the dates of the most 
recent being 1874 and 1882. Venus, like Mercury, appears 
surrounded with a border on passing across the sun's disk, 
as shown in Fig. 262. 




Fig. 262. 



ASTRONOMY. 



235 



Mars. 

235. The Orbit of Mars. — The orbit of Mars is more 
eccentric than that of any of the larger planets, except 
Mercury; its eccentricity being about one-eleventh. The 
inclination of the orbit to the ecliptic is somewhat under 
two degrees. The mean distance of Mars from the sun 
is about a hundred and forty million miles \ but, owing to 
the eccentricity of his orbit, the distance varies from a hun- 
dred and fifty-three million miles to a hundred and twenty- 
seven million miles. 

236. Distance of 
Mars from the 
Earth. — It will be 
seen, from Fig. 263, 
that a superior 
planet (or one out- 
side the orbit of 
the earth), like 
Mars, is nearer the 
earth, by the whole 
diameter of the 
earth's orbit, when 

in opposition than Fig. 263. 

when in conjunction. The mean distance of Mars from 
the earth, at the time of opposition, is a hundred and forty 
million miles minus ninety-two million miles, or forty-eight 
million miles. Owing to the eccentricity of the orbit of 
the earth and of Mars, the distance of this planet when 
in opposition varies considerably. When the earth is in 
aphelion, and Mars in perihelion, at the time of opposition, 
the distance of the planet from the earth is only about 
thirty-three million miles. On the other hand, when the 
earth is in perihelion, and Mars in aphelion, at the time of 
opposition, the distance of the planet is over sixty-two 
million miles. 




236 



ASTRONOMY. 



The mean distance of Mars from the earth when in 
conjunction is a hundred and forty million miles plus 
ninety-two million miles, or two hundred and thirty-two 
million miles. It will therefore be seen that Mars is nearly 
five times as far off at conjunction as at opposition. 




Fig. 264. 

237. The Apparent Size of Mars, — Owing to the vary- 
ing distance of Mars from the earth, the apparent size of 
the planet varies almost as much as that of Venus. Fig. 
264 shows the apparent size of Mars at its extreme and 
mean distances from the earth. The apparent diameter 
varies from about four seconds to about thirty seconds. 




Fig. 265. 

238. The Volujne and Density of Mars. — Among the 
larger planets Mars is next in size to Mercury. Its real 
diameter is somewhat more than four thousand miles, and 
its bulk is about one-seventh of that of the earth. Its size, 
compared with that of the earth, is shown in Fig. 265. 



PLATE IV. 




ASTRONOMY. 237 

The density of Mars is only about three- fourths of that 
of the earth. 

239. Sidereal and Sy nodical Periods of Mars, — The 
sidereal period of Mars, or the time in which he makes a 
complete revolution around the sun, is about six hundred 
and eighty-seven days, or nearly twenty-three months; but 
he is about seven hundred and eighty days in passing from 
opposition to opposition again, or in performing a synodical 
revolution. Mars moves in his orbit at the rate of about 
fifteen miles a second. 

240. Brilliancy of Mars. — When near his opposition, 
Mars is easily recognized with the naked eye by his fiery-red 
light. He is much more brilliant at some oppositions than 
at others, for reasons already explained (236), but always 
shines brighter than an ordinary star of the first magnitude. 

241. Telescopic Appeai'ance of Mars. — When viewed 
with a good telescope (see Plate IV.), Mars is seen to be 
covered with dusky, dull-red patches, which are supposed 
to be continents, like those of our own globe. Other por- 
tions, of a greenish hue, are believed to be tracts of water. 
The ruddy color, which overpowers the green, and makes 
the whole planet seem red to the naked eye, was believed 
by Sir J. Herschel to be due to an ochrey tinge in the 
general soil, like that of the red sandstone districts on the 
earth. In a telescope, Mars appears less red, and the higher 
the power the less the intensity of the color. The disk, 
when well seen, is mapped out in a way which gives at once 
the impression of land and water. The bright part is red in- 
clining to orange, sometimes dotted with brown and greenish 
points. The darker spaces, which vary greatly in depth of 
tone, are of a dull gray-green, having the aspect of a fluid 
which absorbs the solar rays. The proportion of land to 
water on the earth appears to be reversed on Mars. On the 
earth every continent is an island ; on Mars all seas are 
lakes. Long, narrow straits are more common than on the 



238 



ASTRONOMY. 



earth ; and wide expanses of water, like our Atlantic Ocean, 
are rare. (See Fig. 266.) 




Fig. 266. 

Fig. 267 represents a chart of the surface of Mars, which j 




Fig. 267. 



has been constructed from careful telescopic observation. 
The outlines, as seen in the telescope, are, however, much 



ASTRONOMY. 239 

less distinct than they are represented here ; and it is by 
no means certain that the light and dark portions are bodies 
of land and water. 

In the vicinity of the poles brilliant white spots may be 
noticed, which are considered by many astronomers to be 
masses of snow. This conjecture is favored by the fact 
that they appear to diminish under the sun's influence at 
the beginning of the Martial summer, and to increase again 
on the approach of winter. 

242. Rotation of Mars. — On watching Mars with a 
telescope, the spots on the disk are found to move (as 
shown in Fig. 268) in a manner which indicates that the 




Fig. 268. 

planet rotates in about twenty-four hours on an axis in- 
clined about twenty-eight degrees from a perpendicular 
to the plane of its orbit. The inclination of the axis is 
shown in Fig. 269. It is evident from the figure that the 
variation in the length of day and night, and the change 
of seasons, are about the same on Mars as on the earth. 
The changes will, of course, be somewhat greater, and the 
seasons will be about twice as long. 

243. The Satellites of Mars. — In 1877 Professor Hall 
of the Washington Observatory discovered that Mars is 
accompanied by two small moons, whose orbits are shown 
in Fig. 270. The inner satellite has been named Phobos, 
and the outer one Dei?nos. It is estimated that the diame- 
ter of the outer moon is from five to ten miles, and that 
of the inner one from ten to forty miles. 



240 



ASTRONOMY, 






Phobos is remarkable for its nearness to the planet and 
the rapidity of its revolution, which is performed in seven 
hours thirty-eight minutes. Its distance from the centre of 




Fig. 269. 

the planet is about six thousand miles, and from the surface 
less than four thousand. Astronomers on Mars, with tele- 
scopes and eyes like ours, could readily find out whether 




Fig. 270. 

this satellite is inhabited, the distance being less than one- 
sixtieth of that of our moon. 

It will be seen that Phobos makes about three revolutions 






ASTRONOMY. 24.I 

to one rotation of the planet. It will, of course, rise in the 
west ; though the sun, the stars, and the other satellite rise 
in the east. Deimos makes a complete revolution in about 
thirty hours. 

III. THE ASTEROIDS. 

244. BocWs Law of Planetary Distances. — There is a 
yery remarkable law connecting the distances of the planets 
from the sun, which is generally known by the name of 
Bode's Law. Attention was drawn to it in 1778 by the 
astronomer Bode, but he was not really its author. 

To express this law we write the following series of num- 
bers : — 

o, 3, 6, 12, 24, 48, 96; 

each number, with the exception of the first, being double 
the one which precedes it. If we add 4 to each of these 
numbers, the series becomes — , 

4, 7, 10, 16, 28, 52, 100; 
which series was known to Kepler. These numbers, with 
the exception of 28, are sensibly proportional to the dis- 
tances of the principal planets from the sun, the actual 
distances being as follows : — 

Mercury. Venus. Earth. Mars. Jupiter. Saturn. 

3*9 T 2 IO x 5* 2 5 2 '9 95*4 

245. The First Discovery of the Asteroids. — The great 
gap between Mars and Jupiter led astronomers, from the 
time of Kepler, to suspect the existence of an unknown 
planet in this region ; but no such planet was discovered 
till the beginning of the present century. Ceres was dis- 
covered Jan. 1, 1801, Pallas in 1802, Juno in 1804, and 
Vesta in 1807. Then followed a long interval of thirty- 
eight years before Astrcea, the fifth of these minor planets, 
was discovered in 1845. 

246. Olbers's Hypothesis, — After the discovery of Pallas, 



242 ASTRONOMY. 






Olbers suggested his celebrated hypothesis, that the two 
bodies might be fragments of a single planet which had 
been shattered by some explosion. If such were the' case, 
the orbits of all the fragments would at first intersect each 
other at the point where the explosion occurred. He there- 
fore thought it likely that other fragments would be found, 
especially if a search were kept up near the intersection of 
the orbits of Ceres and Pallas. 

Professor Newcomb makes the following observations con- 
cerning this hypothesis : — 

" The question whether these bodies could ever have formed 
a single one has now become one of cosmogony rather than of 
astronomy. If a planet were shattered, the orbit of each frag- 
ment would at first pass through the point at which the explo- 
sion occurred, however widely they might be separated through 
the rest of their course ; but, owing to the secular changes 
produced by the attractions of the other planets, this coinci- 
dence would not continue. The orbits would slowly move 
away, and after the lapse of a few thousand years no trace 
of a common intersection would be seen. It is therefore 
curious that Olbers and his contemporaries should have ex- 
pected to find such a region of intersection, as it implied that 
the explosion had occurred within a few thousand years. The 
fact that the required conditions were not fulfilled was no argu- 
ment against the hypothesis, because the explosion might have 
occurred millions of years ago ; and in the mean time the peri- 
helion and node of each orbit would have made many entire 
revolutions, so that the orbits would have been completely 
mixed up. ... A different explanation of the group is given 
by the nebular hypothesis ; so that Olbers's hypothesis is no 
longer considered by astronomers." 

247. Later Discoveries of Asteroids. — Since 1845 over 
two hundred asteroids have been discovered. All these are 
so small, that it requires a very good telescope to see them ; 
and even in very powerful telescopes they appear as mere 
points of light, which can be distinguished from the stars 
only by their motions. 



ASTRONOMY. 



243 



To facilitate the discovery of these bodies, very accurate 
maps have been constructed, including all the stars down to 
the thirteenth magnitude in the neighborhood of the ecliptic. 
A reduced copy of one of these maps is shown in Fig. 271. 

Furnished with a map of this kind, and with a telescope 
powerful enough to show all the stars marked on it, the 




Fig. 271. 

observer who is searching for these small planets will place 
in the field of view of his telescope six spider-lines at right 
angles to each other, and at equal distances apart, in such 
a manner that several small squares will be formed, embra- 
cing just as much of the heavens as do those shown in the 
map. He will then direct his telescope to the region of the 
sky he wishes to examine, represented by the map, so as to 
be able to compare successively each square with the corre- 



244 



ASTRONOMY. 



sponding portion of the sky. Fig. 272 shows at the right 
hand the squares in the telescopic field of view, and at the 
left hand the corresponding squares of the map. 

He can then assure himself if the numbers and positions of 
the stars mapped, and of the stars observed, are identical. If 
he observes in the field of view a luminous point which is not 
marked in the map, it is evident that either the new body is a 
star of variable brightness which was not visible at the time 




Fig. 272. 

the map was made, or it is a planet, or perhaps a comet. If 
the new body remains fixed at the same point, it is the former; 
but, if it changes its position with regard to the neighboring 
stars, it is the latter. The motion is generally so sensible, that 
in the course of one evening the change of position may be 
detected; and it can soon be determined, by the direction and 
rate of the motion, whether the body is a planet or a comet. 



IV. OUTER GROUP OF PLANETS. 

Jupiter. 

248. Orbit of Jupiter. — The orbit of Jupiter is inclined 
only a little over one degree to the ecliptic ; and its eccen- 
tricity is only about half of that of Mars, being less than 
one-twentieth. The mean distance of Jupiter from the sun 
is about four hundred and eighty million miles ; but, owing 
to the eccentricity of his orbit, his actual distance from the 
sun ranges from four hundred and fifty-seven to five hun- 
dred and three million miles. 



ASTRONOMY. 245 

249. Distance of Jupiter from the Earth. — When 
Jupiter is in opposition, his mean distance from the earth 
is four hundred and eighty million miles minus ninety-two 
million miles, or three hundred and eighty-eight million 
miles, and, when he is in conjunction, four hundred and 
eighty million miles plus ninety- two million miles, or five 
hundred and seventy-two million miles. It will be seen 
that he is less than twice as far off in conjunction as in 
opposition, and that the ratio of his greatest to his least 
distance is very much less than in the case of Venus and 
Mars. This is owing to his very much greater distance from 
the sun. Owing to the eccentricities of the orbits of the 




earth and of Jupiter, the greatest and least distances of 
Jupiter from the earth vary somewhat from year to year. 

250. The Brightness and Apparent Size of Jupiter. — 
The apparent diameter of Jupiter varies from about fifty 
seconds to about thirty seconds. His apparent size at his 
extreme and mean distances from the earth is shown in 
Fig. 273. 

Jupiter shines with a brilliant white light, which exceeds 
that of every other planet except Venus. The planet is, 
of course, brightest when near opposition. 

251. The Volume and Density of Jupiter. — Jupiter is 
the " giant planet " of our system, his mass largely exceed- 
ing that of all the other planets combined. His mean 



246 ASTRONOMY, 






diameter is about eighty-five thousand miles ; but the equa- 
torial exceeds the polar diameter by five thousand miles. 
In volume he exceeds our earth about thirteen hundred 
times, but in mass only about two hundred and thirteen 
times. His specific gravity is, therefore, far less than that 
of the earth, and even less than that of water. The com- 
parative size of Jupiter and the earth is shown in Fig. 274. 

252. The Sidereal and Syno die al Periods of Jupiter. — 
It takes Jupiter nearly twelve years to make a sidereal revo- 




Fig. 274. 

lution, or a complete revolution around the sun, his orbital 
motion being at the rate of about eight miles a second. 
His synodieal period, or the time of his passage from oppo- 
sition to opposition again, is three hundred and ninety-eight 
days. 

253. The Teleseopie Aspeet of Jupiter. — There are no 
really permanent markings on the disk of Jupiter ; but his 
surface presents a very diversified appearance. The earlier 
telescopic observers descried dark belts across it, one north 
of the equator, and the other south of it. With the in- 
crease of telescopic power, it was seen that these bands 



PLATE V, 



ASTRONOMY. 247 

were of a more complex structure than had been supposed, 
and consisted of stratified, cloud-like appearances, varying 
greatly in form and number. These change so rapidly, that 
the face of the planet rarely presents the same appearance 
on two successive nights. They are most strongly marked 
at some distance on each side of the planet's equator, and 
thus appear as two belts under a low magnifying power. 

Both the outlines of the belts, and the color of portions 
of the planet, are subject to considerable changes. The 
equatorial regions, and the spaces between the belts gener- 
ally, are often of a rosy tinge. This color is sometimes 
strongly marked, while at other times hardly a trace of it 
can be seen. A general telescopic view of Jupiter is given 
in Plate V. 

254. The Physical Constitution of "Jupiter. — From the 
changeability of the belts, and of nearly all the visible 
features of Jupiter, it is clear that what we see on that 
planet is not the solid nucleus, but cloud-like formations, 
which cover the entire surface to a great depth. The planet 
appears to be covered with a deep and dense atmosphere, 
filled with thick masses of clouds and vapor. Until recently 
this cloud-laden atmosphere was supposed to be somewhat 
like that of our globe ; but at present the physical constitu- 
tion of Jupiter is believed to resemble that of the sun rather 
than that of the earth. Like the sun, he is brighter in the 
centre than near the edges, as is shown in the transits of 
the satellites over his disk. When the satellite first enters 
on the disk, it commonly seems like a bright spot on a 
dark background; but, as it approaches the centre, it 
appears like a dark spot on the bright surface of the 
planet. The centre is probably two or three times brighter 
than the edges. This may be, as in the case of the sun, 
because the light near the edge passes through a greater 
depth of atmosphere, and is diminished by absorption. 

It has also been suspected that Jupiter shines partly by 



248 ASTRONOMY. 

his own light, and not wholly by reflected sunlight. The 
planet cannot, however, emit any great amount of light; 
for, if it did, the satellites would shine by this light when 
they are in the shadow of the planet, whereas they to- 
tally disappear. It is possible that the brighter portions 
of the surface are from time to time slightly self-lumi- 
nous. 

Again : the interior of Jupiter seems to be the seat of an 
activity so enormous that it can be ascribed only to intense 



Fig. 275. 

heat. Rapid movements are always occurring on his sur- 
face, often changing its aspect in a few hours. It is there- 
fore probable that Jupiter is not yet covered by a solid 
crust, and that the fiery interior, whether liquid or gaseous, 
is surrounded by the dense vapors which cease to be lumi- 
nous on rising into the higher and cooler regions of the 
atmosphere. Figs. 275 and 276 show the disk of Jupiter 
as it appeared in December, 1881. 

255. Rotation of Jupiter. — Spots are sometimes visible 



ASTRONOMY. 



249 



which are much more permanent than the ordinary mark- 
ings on the belts. The most remarkable of these is " the 



Fig. 276. 

great red spot," which was first observed in July, 1878, 
and is still to be seen in February, 1882. It is shown 
just above the centre of the disk in Fig. 275. By watch- 




Fig. 277. 
ing these spots from day to day, the time of Jupiter's axial 
rotation has been found to be about nine hours and fifty 
minutes. 



250 



ASTRONOMY, 



1 



The axis of Jupiter deviates but slightly from a perpen- 
dicular to the plane of its orbit, as is shown in Fig. 277. 



THE SATELLITES OF JUPITER, 

256. Jupiter's Four Moons. — Jupiter is accompanied 




by four moons, as shown in Fig. 278. The diameters of 
these moons range from about twenty-two hundred to thirty- 
seven hundred miles. The second from the planet is the 
smallest, and the third the largest. The smallest is about 
the size of our moon ; the largest considerably exceeds 



r 


^mm*. 






y 






4 


^^^^^M 








•^0^ 


^ART^ \ 



Fig. 279. 

Mercury, and almost rivals Mars, in bulk. The sizes of 
these moons, compared with those of the earth and its 
moon, are shown in Fig. 279. 

The names of these satellites, in the order of their dis- 
tance from the planet, are lo, Europa, Ganymede, and Cat- 



ASTRONOMY. 



2 5 I 



Hi to. Their times of revolution range from about a day 
and three-fourths up to about sixteen days and a half. 
Their orbits are shown in Fig. 280. 

257. The Variability of Jupiter's Satellites. — Remarka- 
ble variations in the light of these moons have led to the 
supposition that violent changes are taking place on their 
surfaces. It was formerly believed, that, like our moon, 




Fig. 280. 

they always present the same face to the planet, and that 
the changes in their brilliancy are due to differences in the 
luminosity of parts of their surface which are successively 
turned towards us during a revolution ; but careful measure- 
ments of their light show that this hypothesis does not 
account for the changes, which are sometimes very sudden. 
The satellites are too distant for examination of their sur- 
faces with the telescope : hence it is impossible to give any 
certain explanation of these phenomena. 



252 



ASTRONOMY. 



258. Eclipses of Jupiter's Satellites. — Jupiter, like the 
earth, casts a shadow away from the sun, as shown in 










111..., 

' . L.- 





Fig. 281. 

Fig. 281 ; and, whenever one of his moons passes into this 
shadow, it becomes eclipsed. On the other hand, whenever 



ASTRONOMY. 



253 



one of the moons throws its shadow on Jupiter, the sun is 
eclipsed to that part of the planet which lies within the 
shadow. 

To the inhabitants of Jupiter (if there are any, and if 





Fig. 282. 

they can see through the clouds) these eclipses must be 
very familiar affairs ; for in consequence of the small incli- 
nations of the orbits of the satellites to the planet's equator, 
and the small inclination of the latter to the plane of 
Jupiter's orbit, all the satellites, except the most distant one, 



254 ASTRONOMY. 






are eclipsed in every revolution. A spectator on Jupiter 
might therefore witness during the planetary year forty-five 
hundred eclipses of the moons, and about the same number 
of the sun. 

259. Transits of Jupiter's Satellites. — Whenever one 
of Jupiter's moons passes in front of the planet, it is said 
to make a transit across his disk. When a moon is making 
a transit, it presents its bright hemisphere towards the earth, 
as will be seen from Fig. 282 : hence it is usually seen as a 
bright spot on the planet's disk : though sometimes, on the 
brighter central portions of the disk, it appears dark. 




Fig. 283. 

It will be seen from Fig. 282 that the shadow of a moon 
does not fall upon the part of the planet's disk that is 
covered by the moon : hence we may observe the transit 
of both the moon and its shadow. The shadow appears 
as a small black spot, which will precede or follow the 
moon according to the position of the earth in its orbit. 
Fig. 283 shows two moons of Jupiter in transit. 

260. Occultations of Jupiter's Satellites. — The eclipse 
of a moon of Jupiter must be carefully distinguished from 
the occultation of a moon by the planet. In the case of 
an eclipse, the moon ceases to be visible, because the mass 



ASTRONOMY. 



255 



of Jupiter is interposed between the sun and the moon, 
which ceases to be luminous, because the sun's light is cut 
off; but, in the case of an occultation, the moon gets into 
such a position that the body of Jupiter is interposed be- 
tween it and the earth, thus rendering the moon invisible 
to us. The third satellite, m" (Fig. 282), is invisible from 
the earth E, having become occulted when it passed behind 
the planet's disk ; but 
it will not be eclipsed 
until it passes into the 
shadow of Jupiter. 

261. jfupiter with o u t 
Satellites. — It occasion- 
ally happens that every 
one of Jupiter's satel- 
lites will disappear at 
the same time, either 
by being eclipsed or 
occulted, or by being in 
transit. In this event, 
Jupiter will appear with- 
out satellites. This oc- 
curred on the 2 1 st of 

August, 1867. The po- Fig. 284. 

sition of Jupiter's satellites at this time is shown in 
Fig. 284. 

Saturn. 
the planet and his moons. 

262. The Orbit of Saturn. — The orbit of Saturn is 
rather more eccentric than that of Jupiter, its eccentricity 
being somewhat more than one-twentieth. Its inclination 
to the ecliptic is about two degrees and a half. The mean 
distance of Saturn from the sun is about eight hundred and 
eighty million miles. It is about a hundred million miles 
nearer the sun at perihelion than at aphelion. 




256 



ASTRONOMY. 



263. Distance of Saturn from the Earth. — The mean 
distance of Saturn from the earth at opposition is eight hun- 
dred and eighty million miles minus ninety-two million 
miles, or seven hundred and eighty-eight million ; and at 
conjunction, eight hundred and eighty million miles plus 
ninety-two million, or nine hundred and seventy-two million. 
Owing to the eccentricity of the orbit of Saturn, his dis- 
tance from the earth at opposition and at conjunction varies 
by about a hundred million miles at different times ; but he 
is so immensely far away, that this is only a small fraction 
of his mean distance. 

264. Apparent Size and Brightness of Saturn. — The 
apparent diameter of Saturn varies from about twenty sec- 
onds to about fourteen seconds. His apparent size at his 




Fig. 285. 

extreme and mean distances from the earth is shown in 
Fig. 285. 

The planet generally shines with the brilliancy of a mod- 
erate first-magnitude star, and with a dingy, reddish light, 
as if seen through a smoky atmosphere. 

265. Volume and Density of Saturn. — The real diame- 
ter of Saturn is about seventy thousand miles, and its 
volume over seven hundred times that of the earth. The 
comparative size of the earth and Saturn is shown in Fig. 
286. This planet is a little more than half as dense as 
Jupiter. 

266. The Sidereal and Synodical Periods of Saturn. — 
Saturn makes a complete revolution round the sun in a 
period of about twenty- nine years and a half, moving in 
his orbit at the rate of about six miles a second. The 



ASTRONOMY. 257 

planet passes from opposition to opposition again in a 
period of three hundred and seventy-eight days, or thirteen 
days over a year. 

267. Physical Constitution of Saturn. — The physical 
constitution of Saturn seems to resemble that of Jupiter; 
but, being twice as far away, the planet cannot be so well 
studied. The farther an object is from the sun, the less it 
is illuminated; and, the farther it is from the earth, the 




Fig. 286. 

smaller it appears : hence there is a double difficulty in 
examining the more distant planets. Under favorable cir- 
cumstances, the surface of Saturn is seen to be diversified 
with very faint markings ; and, with high telescopic powers, 
two or more very faint streaks, or belts, may be discerned 
parallel to its equator. These belts, like those of Jupiter, 
change their aspect from time to time ; but they are so faint 
that the changes cannot be easily followed. It is only on 
■are occasions that the time of rotation can be determined 
:rom a study of the markings. 



258 



ASTRONOMY, 






268. Rotation of Saturn. — On the evening of Dec. 7, 
1876, Professor Hall, who had been observing the satellites 
of Saturn with the great Washington telescope (18), saw a 
brilliant white spot near the equator of the planet. It 
seemed as if an immense eruption of incandescent matter 
had suddenly burst up from the interior. The spot gradu- 
ally spread itself out into a long light streak, of which the 
brightest point was near the western end. It remained visi- 
ble until January, when it became faint and ill-defined, and 
the planet was lost in the rays of the sun. 




Fig. 287. 

From all the observations on this spot, Professor Hall 
found the period of Saturn to be ten hours fourteen minutes, 
reckoning by the brightest part of the streak. Had the 
middle of the streak been taken, the time would have been 
less, because the bright matter seemed to be carried along 
in the direction of the planet's rotation. If this motion 
was due to a wind, the velocity of the current must have 
been between fifty and a hundred miles an hour. The axis 
of Saturn is inclined twenty-seven degrees from the per- 
pendicular to its orbit. 



ASTRONOMY. 



259 



269. The Satellites of Saturn. — Saturn is accompanied 
by eight moons. Seven of these are shown in Fig. 287. 
The names of these satellites, in the order of their distances 
from the planet, are given in the accompanying table : — 







Distance 








1 


NAME. 


from 
Planet in 


Sidereal Period. 


Discoverer. 


Date of Discovery. 


9 

z 




Miles. 














d. h. m. 


d. 






I 


Mimas . . 


120,800 


22 27 


0.94 


Herschel . . 


Sept. 17, 1789. 


2 


Enceladus . 


155,000 


1 8 53 


i-37 


Herschel . . 


Aug. 28, 1789. 


3 


Tethys . . 


191,900 


1 21 18 


1.88 


Cassini . . 


March, 1684. 


4 


Dione . . . 


245,800 


2 17 41 


2-73 


Cassini . . 


March, 1684. 


5 


Rhea . . . 


343>4oo 


4 12 25 


4.51 


Cassini . . 


Dec. 23, 1672. 


6 


Titan . . . 


796,100 


15 22 41 


15-94 


Huyghens . 


March 25, 1655. 


7 


Hyperion 


963,300 


21 7 7 


21.29 


Bond . . . 


Sept. 16, 1848. 


8 


Japetus . . 


2,313,800 


79 7 53 


79-33 


Cassini . . 


October, 1671. 



The apparent brightness or visibility of these satellites 
follows the order of their discovery. The smallest telescope 
will show Titan, and one of very moderate size will show 
Japetus in the western part of its orbit. An instrument of 
four or five inches aperture will show Rhea, and perhaps 
Tethys and Dione ; while seven or eight inches are required 
for Enceladus, even at its greatest elongation from the planet. 




Mimas can rarely be seen except at its greatest elongation, and 
then only with an aperture of twelve inches or more. Hype- 
rion can be detected only.with the most powerful telescopes, 
on account of its faintness and the difficulty of distinguishing 
it from minute stars. 

Japetus, the outermost satellite, is remarkable for the fact, 
that while, in one part of its orbit, it is the brightest of the 
satellites except Titan, in the opposite part it is almost as 



26o 



ASTRONOMY. 




Fig. 289. 



ASTRONOMY. 



26l 



faint as Hyperion, and can be seen only in large telescopes. 
When west of the planet, it is bright; when east of it, faint. 
This peculiarity has been accounted for by supposing that the 
satellite, like our moon, always presents the same face to the 
planet and that one side of it is white and the other intensely 
black; but it is doubtful whether any known substance is so 
black as one side of the satellite must be to account for such 
extraordinary changes of brilliancy. 

Titan, the largest of these satellites, is about the size 
of the largest satellite of Jupiter. The relative sizes of 




the satellites are shown 
Fig. 289. 

Fig. 290 shows the transit of one of the satellites, and 
of its shadow, across the disk of the planet. 

THE RIXGS OF SATURN. 

270. General Appearance of the Rings. — Saturn is sur- 
rounded by a thin flat ring lying in the plane of its equator. 
This ring is probably less than a hundred miles thick. The 
part of it nearest Saturn reflects little sunlight to us : so that 
it has a dusky appearance, and is not easily seen, although 
it is not quite so dark as the sky seen between it and the 
planet. The outer edge of this dusky portion of the ring 
is at a distance from Saturn of between two and three 
times the earth's diameter. Outside of this dusky part of 



262 



ASTRONOMY. 




Fig. 291. 



ASTRONOMY. 



263 



the ring is a much brighter portion, and outside of this 
another, which is somewhat fainter, but still so much brighter 
than the dusky part as to be easily seen. The width of the 
brighter parts of the ring is over three times the earth's 
diameter. To distinguish the parts, the outer one is called 
ring A, the middle one ring B, and the dusky one ring C. 
Between A and B is an apparently open space, nearly two 
thousand miles wide, which looks like a black line on the 
ring. Other divisions in the ring have been noticed at 
times ; but this is the only one always seen with good tele- 
scopes at times when either side of the ring is in view from 
the earth. The general telescopic appearance of the ring 
is shown in Fig. 291. 




Fig. 292. 

Fig. 292 shows the divisions of the rings as they were 
seen by Bond. 

271. Phases of Saturn's Ring. — The ring is inclined 
to the plane of the planet's orbit by an angle of twenty- 
seven degrees. The general aspect from the earth is nearly 
the same as from the sun. As the planet revolves around 
the sun, the axis and plane of the ring keep the same 
direction in space, just as the axis of the earth and the 
plane of the equator do. 

When the planet is in one part of its orbit, we see the 



264 



ASTRONOMY. 






upper or northern side of the ring at an inclination of 
twenty-seven degrees, the greatest angle at which the ring 
can ever be seen. This phase of the ring is shown in 
Fig. 293. 




Fig. 293. 

When the planet has moved through a quarter of a 
revolution, the edge of the ring is turned towards the sun 
and the earth ; and, owing to its extreme thinness, it is 
visible only in the most powerful telescopes as a fine line 




This 



Fig. 294. 

of light, stretching out on each side of the planet, 
phase of the ring is shown in Fig. 294. 

All the satellites, except Japetus, revolve very nearly in 
the plane of the ring : consequently, when the edge of the 
ring is turned towards the earth, the satellites seem to swing 



ASTRONOMY. 



265 



from one side of the planet to the other in a straight line, 
running along the thin edge of the ring like beads on a 
string. This phase affords the best opportunity of seeing 




Fig. 295. 

the inner satellites, Mimas and Enceladus, which at other 
times are obscured by the brilliancy of the ring. 




Fig. 296. 



Fig. 295 shows a phase of the ring intermediate between 
the last two. 
When the planet has moved ninety degrees farther, we 



266 



ASTRONOMY. 



again see the ring at an angle of twenty-seven degrees ; but 
now it is the lower or southern side which is visible. When 
it has moved ninety degrees farther, the edge of the ring 
is again turned towards the earth and sun. 

The successive phases of Saturn's ring during a complete 
revolution are shown in Fig. 296. 




Fig. 297. 

It will be seen that there are two opposite points of 
Saturn's orbit in which the rings are turned edgewise tc 
us, and two points half-way between the former in whicr 
the ring is seen at its maximum inclination of about twenty- 
seven degrees. Since the planet performs a revolution ir 
twenty-nine years and a half, these phases occur at average 
intervals of about seven years and four months. 






ASTRONOMY. 267 

272. Disappearance of Saturn's Ring. — It will be seen 
from Fig. 297 that the plane of the ring may not be turned 
towards the sun and the earth at exactly the same time, and 
also that the earth may sometimes come on one side of the 
plane of the ring while the sun is shining on the other. In 
the figure, E, E', E'\ and E'" is the orbit of the earth. 
When Saturn is at S', or opposite, at E, the plane of the ring 
will pass through the sun, and then only the edge of the ring 
will be illumined. Were Saturn at S, and the earth at E', the 
plane of the ring would pass through the earth. This would 
also be the case were the earth at E'", and Saturn at S". 




Fig. 298. 

Were Saturn at S or at S /f , and the earth farther to the left or 
to the right, the sun would be shining on one side of the ring 
while we should be looking on the other. In all these cases 
the ring will disappear entirely in a telescope of ordinary 
power. With very powerful telescopes the ring will appear, in 
the first two cases, as a thin line of light (Fig. 298). It will 
be seen that all these cases of disappearance must take place 
when Saturn is in the parts of his orbit intercepted between 
the parallel lines A C and BD. These lines are tangent to 
the earth's orbit, which they enclose, and are parallel to the 
plane of Saturn's ring. As Saturn passes away from these 
two lines on either side, the rings appear more and more open. 
When the dark side of the ring is in view, it appears as a 



268 ASTRONOMY. 

black line crossing the planet ; and on such occasions the sun- 
light reflected from the outer and inner edges of the rings A 
and B enables us to see traces of the ring on each side of 
Saturn, at least in places where two such reflections come 
nearly together. Fig. 299 illustrates this reflection from the 
edges at the divisions of the rings. 

273. Changes in Saturn's Ring. — The question whether 
changes are going on in the rings of Saturn is still unsettled. 
Some observers have believed that they saw additional divis- 
ions in the rings from time to time ; but these may have been 
errors of vision, due partly to the shading which is known to 
exist on portions of the ring. 




Fig. 299. 

Professor Newcomb says, '-As seen with the great Wash- 
ington equatorial in the autumn of 1874, there was no great or 
sudden contrast between the inner or dark edge of the bright 
ring and the outer edge of the dusky ring. There was some 
suspicion that the one shaded into the other by insensible 
gradations. Xo one could for a moment suppose, as some 
observers have, that there was a separation between these two 
rings. All these considerations give rise to the question 
whether the dusky ring may not be growing at the expense 
of the inner bright ring." 

Struve, in 1851, advanced the startling theory that the inner 
edge of the ring was gradually approaching the planet, the 



ASTRONOMY. 269 

whole ring spreading inwards, and making the central opening 
smaller. The theory was based upon the descriptions and 
drawings of the rings by the astronomers of the seventeenth 
century, especially Huyghens, and the measures made by later 
astronomers up to 1851. This supposed change in the dimen- 
sion of the ring is shown in Fig. 300. 

274. Constitution of Saturn's Ring. — The theory now gen- 
erally held by astronomers is, that the ring is composed of a 
cloud of satellites too small to be separately seen in the tele- 
scope, and too close together to admit of visible intervals 
between them. The ring looks solid, because its parts are 
tco small and too numerous to be seen singly. They are like 
the minute drops of water that make up clouds and fogs, 
which to our eyes seem like solid masses. In the dusky ring 
the particles may be so scattered that we can see through 




Fig. 300. 

the cloud, the duskiness being due to the blending of light and 
darkness. Some believe, however, that the duskiness is caused 
by the darker color of the particles rather than by their being 
farther apart. 

Uranus. 

275. Orbit and Dimensions of Uranus. — Uranus, the 
smallest of the outer group of planets, has a diameter of 
nearly thirty-two thousand miles. It is a little less dense 
than Jupiter, and its mean distance from the sun is about 
seventeen hundred and seventy millions of miles. Its orbit 
has about the same eccentricity as that of Jupiter, and is 
inclined less than a degree to the ecliptic. Uranus makes 



270 



ASTRONOMY. 




a revolution around the sun in eighty-four years, moving at 
the rate of a little over four miles a second. It is visible 
to the naked eye as a star of the sixth magnitude. 

As seen in a large telescope, the planet has a decidedly 
sea-green color : but no markings have with certainty been 

detected on its disk, so that 
nothing is really known with 
regard to its rotation. Fig. 
301 shows the comparative 
size of Uranus and the 
earth. 

276. Discovery of Uranus. 
— This planet was discovered 
by Sir William Herschel in March. 1781. He was engaged 
at the time in examining the small stars of the constellation 
Gemini, or the Twins. He noticed that this object which 
had attracted his attention had an appreciable disk, and 
therefore could not be a star. He also perceived by its 
motion that it could not be 
a nebula : he therefore con- 
cluded that it was a comet, 
and announced his discovery 
as such. On attempting to 
compute its orbit, it was 
soon found that its motions 
could be accounted for only 
on the supposition that it 
was moving in a circular 
orbit at about twice the dis- 
tance of Saturn from the Fi -- 3° 2 - 
sun. It was therefore recognized as a new planet, whose 
discovery nearly doubled the dimensions of the solar system 
as it was then known. 

277. The Name of the Planet. — Herschel. out of compli 

merit to his patron. George III., proposed to call the new 




ASTRONOMY. 27 1 

planet Georgium Sidus (the Georgian Star); but this name 
found little favor. The name of Herschel was proposed, and 
continued in use in England for a time, but did not meet with 
o-eneral approval. Various other names were suggested, and 
finally that of Uranus was adopted. 

278. The Satellites of Uranus. — Uranus is accompanied 
by four satellites, whose orbits are shown in Fig. 302. These 
satellites are remarkable for the great inclination of their 
orbits to the plane of the planet's orbit, amounting to about 
eighty degrees, and for their retrograde motion ; that is, 
they move from east to west, instead of from west to east, 
as in the case of all the planets and of all the satellites 
previously discovered. 

Neptune. 

279. Orbit and Dimensions of Neptune. — So far as 
known, Neptune is the most remote member of the solar 
system, its mean distance from the sun being twenty-seven 
hundred and seventy-five million miles. This distance is 
considerably less than twice that of Uranus. Neptune 
revolves around the sun in a period of a little less than 
a hundred and sixty-five years. Its orbit has but slight 
eccentricity, and is inclined less than two degrees to the 
ecliptic. This planet is considerably larger than Uranus, 
its diameter being nearly thirty-five thousand miles. It is 
somewhat less dense than Uranus. Neptune is invisible to 
the naked eye, and no telescope has revealed any markings 
on its disk: hence nothing is certainly known as to its 
rotation. Fig. 303 shows the comparative size of Neptune 
and the earth. 

280. The Discovery of Neptune. — The discovery of 
Neptune was made in 1846, and is justly regarded as one 
of the grandest triumphs of astronomy. 

Soon after Uranus was discovered, certain irregularities in 
its motion were observed, which could not be explained. It 



272 



ASTRONOMY.. 



is well known that the planets are all the while disturbing 
each other's motions, so that none of them describe perfect 
ellipses. These mutual disturbances are called perturba- 
tions. In the case of Uranus it was found, that, after 
making due allowance for the action of all the known 
planets, there were still certain perturbations in its course 
which had not been accounted for. This led astronomers 
to the suspicion that these might be caused by an unknown 
planet. Leverrier in France, and Adams in England, inde- 
pendently of each other, set themselves the difficult problem 
of computing the position and magnitude of a planet which 
would produce these perturbations. Both, by a most labo- 
rious computation, showed that the 
perturbations were such as would be 
produced by a planet revolving about 
the sun at about twice the distance 
of Uranus, and having a mass some- 
what greater than that of this planet j 
and both pointed out the same part 
Fl s- 3°3- of the heavens as that in which the 

planet ought to be found at that time. Almost immedi- 
ately after they had announced the conclusion to which 
they had arrived, the planet was found with the tele- 
scope. The astronomer who was searching for the planet 
at the suggestion of Leverrier was the first to recognize 
it : hence Leverrier has obtained the chief credit of the 
discovery. 

The observed planet is proved to be nearer than the one 
predicted by Leverrier and Adams, and therefore of smaller 
magnitude. 

281. The Observed Planet not the Predicted One. — 'Ptth 
fessor Peirce always maintained that the planet found by obser- 
vation was not the one whose existence had been predicted by 
Leverrier and Adams, though its action would completely ex- 
plain all the irregularities in the motion of Uranus. His last 




ASTRONOMY. 



273 



statement on this point is as follows : " My position is, that 
there were two possible planets, either of which might have 
caused the observed irregular 
motions of Uranus. Each 
planet excluded the other; so 
that, if one was, the other was 
not. They coincided in direc- 
tion from the earth at certain 
epochs, once in six hundred 
and fifty years. It was at one 
of these epochs that the pre- 
diction w r as made, and at no 
other time for six centuries 
could the prediction of the 
one planet have revealed the 
other. The observed planet was not the predicted one." 

282. Bode's Law Disproved, — The following table gives 
the distances of the planets according to Bode's law, their 
actual distances, and the error of the law in each case : — 




Planet. 


Numbers of Bode. 


Actual 
Distances. 


Errors. 


Mercury .... 

Venus 

Earth 

Mars 

Minor planets . . 

Jupiter 

Saturn 

Uranus 

Xeptune .... 


+ 4= 4 

3 + 4= 7 

6 + 4 = 10 

12 -f 4 = 16 

24 4- 4 — 28 
48 + 4 = 5 2 

96 -f 4 — TOO 

192 + 4 = 196 
384 4- 4 = 388 


3-9 

7.2 

10. 

15.2 

20 to 35 

52.0 

95-4 
191.9 

3006 


O.I 

0.2 
O.O 

0.8 

0.0 
4.6 
4.1 

87.4 



It will be seen, that, before the discovery of Neptune, the 
agreement was so close as to indicate that this was an actual 
law of the distances ; but the discovery of this planet com- 
pletely disproved its existence. 



274 



ASTRONOMY. 






283. The Satellite of Neptune. — Neptune is accompa- 
nied by at least one moon, whose orbit is shown in Fig. 
304. The orbit of this satellite is inclined about thirty 
degrees to the plane of the ecliptic, and the motion of the 
satellite is retrograde, or from east to west. 

VII. COMETS AND METEORS. 
I. COMETS. 

General Phenomena of Comets. 

284. General Appearance of a Bright Comet. — Comets 
bright enough to be seen with the naked eye are composed 
of three parts, which run into each other by insensible 

gradations. These 
are the nucleus, the 
coma, and the tail 

The nucleus is the 
bright centre of the 
comet, and appears 
to the eye as a 
star or planet. 

The coma is a 
nebulous mass sur- 
rounding the nu- 
cleus on all sides.. 
Close to the nucleus 
it is almost as bright 
as the nucleus it- 
self; but it gradually 
shades off in every direction. The nucleus and coma com- 
bined appear like a star shining through a small patch of 
fog ; and these two together form what is called the head 
of the comet. 

The tail is a continuation of the coma, and consists of a 




Fig. 305. 



ASTRONOMY. 275 

stream of milky light, growing wider and fainter as it 
recedes from the head, till the eye is unable to trace it. 

The general appearance of one of the smaller of the 
brilliant comets is shown in Fig. 305. 







II Bllte "^ 
























•'- ;■•. ■ - _ : ; ; :. ;; . -. 


























t?T * £" , -~ 


"f|^M 


. :";- ' 




' -ifl 





Fig. 306. 

285. General Appearance of a Telescopic Comet. — The 
great majority of comets are too faint to be visible with the 
naked eye, and are called 
telescopic comets. In these 
comets there seems to be 
a development of coma 
at the expense of nucleus 
and tail. In some cases 
the telescope fails to re- 
veal any nucleus at all in 
one of these comets ; at 
other times the nucleus is 
so faint and ill-defined as 
to be barely distinguisha- 
ble. Fig. 306 shows a 
telescopic comet without 
any nucleus at all, and Fi s- 307. 

another with a slight condensation at the centre. In these 
comets it is generally impossible to distinguish the coma 
from the tail, the latter being either entirely invisible, as in 




276 ASTRONOMY. 

Fig. 306, or else only an elongation of the coma, as shown 




Fig. 308. 

in Fig. 307. Many comets appear simply as patches of 




Fig. 309. 

foggy light of more or less irregular form. 



ASTRONOMY. 



277 




286. The Development of Telescopic Comets on their 
Approach to the Sun. — As a rule, all comets look nearly 
alike when they first come within the reach of the tele- 
scope. They appear at first as little foggy patches, with- 
out any tail, and often with- 
out any visible nucleus. As 
they approach the sun their 
peculiarities are rapidly de- 
veloped. Fig. 308 shows 
such a comet as first seen, 
and the gradual development 
of its nucleus, head, and 
tail, as it approaches the 
sun. 

If the comet is only a 
small one, the tail developed 
is small ; but these small appendages have a great variety 
of form in different comets. Fig. 309 shows the singular 
form into which Encke's comet was developed in 18 71. 

Figs. 310 and 311 show 
other peculiar developments 
of telescopic comets. 

287. D eve lop m en t of Bril- 
liant Comets on their Ap- 
proach to the Sun. — Bril- 
liant comets, as well as 
telescopic comets, appear 
nearly alike when they come 
into the view of the tele- 
scope ; and it is only on 
Fi s- 3". their approach to the sun 

that their distinctive features are developed. Not only do 
these comets, when they first come into view, resemble 
each other, but they also bear a close resemblance to tele- 
scopic comets. 




278 ASTRONOMY. 






As the comet approaches the sun, bright vaporous jets, 
two or three in number, are emitted from the nucleus on 
the side of the sun and in the direction of the sun. These 
jets, though directed towards the sun, are soon more or less 
carried backward, as if repelled by the sun. Fig. 312 
shows a succession of views of these jets as they were 
developed in the case of Halley's comet in 1835. 




Fig. 312. 

The jets in this case seemed to have an oscillatory motion. 
At 1 and 2 they seemed to be attracted towards the sun, 
and in 3 to be repelled by him. In 4 and 5 they seemed 
to be again attracted, and in 6 to be repelled, but in a 
reverse direction to that in 3. In 7 they appeared to be 
again attracted. Bessel likened this oscillation of the jets 
to the vibration of a magnetic needle when presented to 
the pole of a magnet. 

In the case of larger comets these luminous jets are sur- 



ASTRONOMY. 



279 



rounded by one or more envelops, which are thrown off in 
succession as the comet approaches the sun. The forma- 
tion of these envelops was a conspicuous feature of Donates 
comet of 1858. A rough view of the jets and the surround- 
ing envelops is given in Fig. 313. Fig. 314 gives a view 
of the envelops without the jets. 

288. The Tails of Comets. — The tails of brilliant comets 
are rapidly formed as the comet approaches the sun, their 
increase in length often 
being at the rate of several 
million miles a day. These 
appendages seem to be 
formed entirely out of the 
matter which is emitted 
from the nucleus in the 
luminous jets which are 
at first directed towards 
the sun. The tails of 
comets are, however, always 
directed away from the sun, 
as shown in Fig. 315. 

It will be seen that the 
comet, as it approaches the 
sun, travels head foremost ; 
but as it leaves the sun it 
goes tail foremost. Fi £- 3*3- 

The apparent length of the tail of a comet depends 
partly upon its real length, partly upon the distance of the 
comet, and partly upon the direction of the axis of the tail 
with reference to the line of vision. The longer the tail, 
the nearer the comet ; and the more nearly at right angles 
to the line of vision is the axis of the tail, the greater is 
the apparent length of the tail. In the majority of cases 
the tails of comets measure only a few degrees ; but, in the 
case of many comets recorded in history, the tail has ex- 
tended half way across the heavens. 




28o 



ASTRONOMY. 






The tail of a comet, when seen at all, is usually several 
million miles in length ; and in some instances the tail is 
long enough to reach across the orbit of the earth, or twice 
as far as from the earth to the sun. 

The tails of comets are apparently hollow, and are some- 
times a million of miles in diameter. So great, however, 
is the tenuity of the matter in them, that the faintest stars 




Fig. 314. 



See 



are seen through it without any apparent obscuration. 
Fig. 316, which is a view of the great comet of 1264. 

The tails of comets are sometimes straight, as in Fig. 
316, but usually more or less curved, as in Fig. 317, which 
is a view of Donates comet as it appeared at one time. 
The tail of a comet is occasionally divided into a number 
of streamers, as in Figs. 318 and 319. Frg. 318 is a 
view of the great comet of 1744, and Fig. 319 of the 



ASTRONOMY, 



28l 



great comet of 1861. No. 1, in Fig. 320, is a view of the 
comet of 1577; No. 2, of the comet of 1680; and No. 3, 
of the comet of 1769. 

Fig. 321 shows some of the 
forms which the imagination 
of a superstitious age saw de- 
picted in comets, when these 
heavenly visitants were thought 
to be the forerunners of wars, 
pestilence, famine, and other 
dire calamities. 

289. Visibility of Comets. — 
Even the brightest comets are 
visible only a short time near 
their perihelion passage. When 
near the sun, they sometimes 
become very brilliant, and on 
rare occasions have been visible, 
even at mid-day. It is seldom 
that a comet can be seen, even 
with a powerful telescope, dur- 
ing its perihelion passage, un- 
less its perihelion is either in- 
side of the earth's orbit, or 
but little outside of it. 



Fig. 315- 




Motion and Origin of Comets. 

290. Recognition of a Telescopic Comet. — It is impossi- 
ble to distinguish telescopic comets by their appearance 
from another class of heavenly bodies known as nebula. 
Such comets can be recognized only by their motion. 



282 



ASTRONOMY. 



Thus, in Fig. 322, the upper and lower bodies look exactly 
alike ; but the upper one is found to remain stationary, 
while the lower one moves across the field of view. The 
upper one is thus shown to be a nebula, and the lower 
one a comet. 

291. Orbits of Comets. — All comets are found to move 
in very eccentric ellipses, in parabolas, or in hypei'bolas. 

Since an ellipse is a 
closed curve (48), all com- 
ets that move in ellipses, 
no matter how eccentric, 
are permanent members 
of the solar system, and 
will return to the sun at 
intervals of greater or less 
length, according to the 
size of the ellipses and 
the rate of the comet's 
motion. 

Parabolas and hyperbo- 
las being open curves (48), 
comets that move in either 
of these orbits are only 
temporary members of our 
solar system. After pass- 
Fig. 316. ing the sun, they move 
off into space, never to return, unless deflected hither by 
the action of some heavenly body which they pass in their 
journey. 

Since a comet is visible only while it is near the sun, it is 
impossible to tell, by the form of the portion of the orbit 
which it describes during the period of its visibility, whether 
it is a part of a very elongated ellipse, a parabola, or a hyper- 
bola. Thus in Fig. 323 are shown two orbits, one of which 
is a very elongated ellipse, and the other a parabola. The 




ASTRONOMY. 



283 




Fig- 3*7- 

part a&, in each case, is the portion of the orbit described 
by the comet during its visibility. While describing the dotted 




Fig. 318. 

portions of the orbit, the comet is invisible. Now it is impos- 



284 ASTRONOMY. 

sible to distinguish the form of the visible portion in the 
two orbits. The same would be true were one of the orbits 
a hyperbola. 

Whether a comet will describe an ellipse, a parabola, or a 
hyperbola, can be determined only by its velocity, taken in con- 
nection with its distance from the sun. Were a comet ninety- 
two and a half million miles from the sun, moving away from 
the sun at the rate of twenty-six miles a second, it would have 
just the velocity necessary to describe a pai'abola. Were it 
moving with a greater velocity, it would necessarilv describe 




Fig. 319. 

a hyperbola, and, with a less velocity, an ellipse. So, at any 
distance from the sun, there is a certain velocity which would 
cause a comet to describe a parabola ; while a greater velocity 
would cause it to describe a hyperbola, and a less velocity to 
describe an ellipse. If the comet is moving in an ellipse, the 
less its velocity, the less the eccentricity of its orbit : hence, in 
order to determine the form of the orbit of any comet, it is 
only necessary to ascertain its distance from the sun, and its 
velocity at any given time. 



ASTRONOMY. 



285 




Fig. 320. 

Comets move in every direction in their orbits, and these 
rbits have every conceivable inclination to the ecliptic. 



286 



ASTRONOMY. 




11 



Fig. 321. 



292. Periodic Cornets. — There are quite a number of 



ASTRONOMY. 



287 



comets which are known to be periodic, returning to the 
sun at regular intervals in elliptic orbits. Some of these 
have been observed at sev- 



eral returns, so that their 
period has been determined 
with great certainty. In the 
case of others the perio- 
dicity is inferred from the 
fact that the velocity fell 
so far short of the parabolic 
limit that the comet must 
move in an ellipse. The 
number of known periodic 
comets is increasing every Fig. 322. 

year, three having been added to the list in 1881. 

The velocity of most comets is so near the parabolic limit 
that it is not possible to decide, from observations, whether it 




falls short of it, or exceeds it. 



Sun 



Fig. 323. 



'In the case of a few comets 
the observations indi- 
cate a minute excess of 
velocity : but this cannot 
{ be confidently asserted. 
v It is not. therefore, abso- 
1 lutely certain that any 
I known comet revolves 
! in a hyperbolic orbit: 
] and thus it is possible 
,' that all comets belong 
/ to our system, and will 
1 ultimately return to it. 

r J 

It is, however, certain, 
that, in the majority of 
cases, the return will be 
delayed for many cen- 



turies, and perhaps for many thousand years. 

293. Origin of Comets. — It is now generally believed that 
the original home of the comets is in the stellar spaces outside 



288 ASTRONOMY. 






of our solar system, and that they are drawn towards the sun, 
one by one, in the long lapse of ages. Were the sun unaccom- 
panied by planets, or were the planets immovable, a comet 
thus drawn in would whirl around the sun in a parabolic orbit, 
and leave it again never to return, unless its path were again 
deflected by its approach to some star. But, when a comet is 
moving in a parabola, the slightest retardation would change 
its orbit to an ellipse, and the slightest acceleration into a 
hyperbola. Owing to the motion of the several planets in 
their orbits, the velocity of a comet would be changed on 
passing each of them. Whether its velocity would be acceler- 
ated or retarded, would depend upon the way in which it passed. 
Were the comet accelerated by the action of the planets, on 
its passage through our system, more than it was retarded by 
them, it would leave the system with a more than parabolic 
orbit, and would therefore move in a hyperbola. Were it, on 
the contrary, retarded more than accelerated by the action of 
the planets, its velocity would be reduced, so that the comet 
would move in a more or less elongated ellipse, and thus 
become a permanent member of the solar system. 

In the majority of cases the retardation would be so slight 
that it could not be detected by the most delicate observation, 
and the comet would return to the sun only after the expiration 
of tens or hundreds of thousands of years ; but, were the 
comet to pass very near one of the larger planets, the retarda- 
tion might be sufficient to cause the comet to revolve in an 
elliptical orbit of quite a short period. The orbit of a comet 
thus captured by a planet would have its aphelion point near 
the orbit of the planet which captured it. Now, it happens 
that each of the larger planets has a family of comets whose 
aphelia are about its own distance from the sun. It is there- 
fore probable that these comets have been captured by the 
action of these planets. As might be expected from the gigan- 
tic size of Jupiter, the Jovian family of comets is the largest. 
The orbits of several of the comets of this group are shown 
in Fig. 324. 

294. Member of Comets. — The number of comets re- 
corded as visible to the naked eye since the birth of Christ 



ASTRONOMY. 



289 



is about five hundred, while about two hundred telescopic 
comets have been observed since the invention of the tele- 
scope. The total number of comets observed since the 
Christian era is therefore about seven hundred. It is cer- 
tain, however, that only an insignificant fraction of all exist- 
ing comets have ever been observed. Since they can be 




Fig. 324. 

seen only when near their perihelion, and since it is probable 
that the period of most of those which have been observed 
is reckoned by thousands of years (if, indeed, they ever 
return at all), our observations must be continued for many 
thousand years before we have seen all which come within 
range of our telescopes. Besides, as already stated (289), 
a comet can seldom be seen unless its perihelion is either 



290 



ASTRONOMY. 



inside the orbit of the earth, or but little outside of it ; and 
it is probable that the perihelia of the great majority of 
comets are beyond this limit of visibility. 

Remarkable Comets. 

295. The Comet of 1680. — The great comet of 1680, shown 
in Fig. 320, is one of the most celebrated on record. It was 
by his study of its motions that Newton proved the orbit of 
a comet to be one of the conic sections, and therefore that 
these bodies move under the influence of gravity. This comet 
descended almost in a direct line to the sun, passing nearer to 
that luminary than any comet before known. Newton esti- 
mated, that, at its perihelion point, it was exposed to a tempera- 
ture two thousand times that of red-hot iron. During its 
perihelion passage it was exceedingly brilliant. Halley sus- 
pected that this comet had a period of five hundred and 
seventy-five years, and that its first recorded appearance was 
in 43 B.C., its third in 1106, and its fourth in 1680. If this is 
its real period, it will return in 2255. The comet of 43 B.C. 
made its appearance just after the assassination of Julius 
Caesar. The Romans called it the Julian Star, and regarded 
it as a celestial chariot sent to convey the soul of Caesar to the 
skies. It was seen two or three hours before sunset, and con- 
tinued visible for eight successive days. The great comet of 
1 106 was described as an object of terrific splendor, and was 
visible in close proximity to the sun. The comet of 1680 has 
become celebrated, not only on account of its great brilliance, 
and on account of Newton's investigation of its orbit, but also 
on account of the speculation of the theologian Whiston in 
regard to it. He accepted five hundred and seventy-five years 
as its period, and calculated that one of its earlier apparitions 
must have occurred at the date of the flood, which he supposed 
to have been caused by its near approach to the earth ; and he 
imagined that the earth is doomed to be destroyed by fire on 
some future encounter with this comet. 

296. The Comet of 1811. — The great comet of 181 1, a view 
of which is given in Fig. 325, is, perhaps, the most remarkable 
comet on record. It was visible for nearly seventeen months, 



ASTRONOMY. 



29I 



and was very brilliant, although at its perihelion passage it 
was over a hundred million miles from the sun. Its tail was 
a hundred and twenty million miles in length, and several 
million miles through. It has been calculated that its aphelion 
point is about two hundred times as far from the sun as its 
perihelion point, or some seven times the distance of Neptune 
from the sun. Its period is estimated at about three thousand 
years. It was an object" of superstitious terror, especially in 
the East. The Russians regarded it as presaging Napoleon's 
great and fatal war with Russia. 




297. Halley's Coitiet. — Halley : s comet has become one of 
the most celebrated of modern times. It is the first comet 
whose return was both predicted and observed. It made its 
appearance in 1682. Halley computed its orbit, and compared 
it with those of previous comets, whose orbits he also com- 
puted from recorded observations. He found that it coincided 
so exactly with that of the comet observed by Kepler in 1607, 
that there could be no doubt of the identity of the two orbits. 
So close were they together, that, were they both drawn in the 



292 



ASTRONOMY. 






heavens, the naked eve would almost see them joined into one 
line. There could therefore be no doubt that the comet of 
1682 was the same that had appeared in 1607. and that it moved 
in an elliptic orbit, with a period of about seventy-five years. 

He found that this 
comet had previ- 
ously appeared in 
1 53 1 and in 1456; 
and he predicted 
that it would return 
about 1758. Its 
actual return was 
retarded somewhat 
by the action of 
the planets on it in 
its passage through 
the solar system. 
It, however, ap- 
Fig. 326. peared again in 

1759, an d a third time in 1835. ^ ts next appearance will be 
about 191 1. The orbit of this comet is shown in Fig. 326. 
Fig. 327 shows the comet as it appeared to the naked eye, and 



1 ( 


/d&*\ \ \ 


\ m 


E AJJTH \ \ 


P 










rvJjPilS^f / J j 


I \ \ 




\ \> 




\ V^ 






\ ^j 






\v 1 






\Oa 






^£oje 


w 




137 

i 


\y 




Fig. 327. 

in a telescope of moderate power, in 1835. This comet appears 
to be growing less brilliant. In 1456 it appeared as a comet 
of great splendor ; and coming as it did in a very superstitious 
age, soon after the fall of Constantinople, and during the threat- 



ASTRONOMY. 



293 



ened invasion of Europe by the Turks, it caused great alarm. 
Fig. 328 shows the changes undergone by the nucleus of this 
comet during its perihelion passage in 1835. 

298. Encke's Comet. — This telescopic comet, two views of 
which are given in Figs. 329 and 330, appeared in 1818. Encke 
computed its orbit, and found it to lie wholly within the orbit 
of Jupiter (Fig. 324), and the period to be about three years 
and a third. By comparing the intervals between the succes- 




Fig. 328. 

sive returns of this comet, it has been ascertained that its 
orbit is continually growing smaller and smaller. To account 
for the retardation of this comet, Olbers announced his cele- 
brated hypothesis, that the celestial spaces are filled with a 
subtile resisting medium. This hypothesis was adopted by 
Encke, and has been accepted by certain other astronomers ; 
but it has by no means gained universal assent. 

299. Biela^s Comet, — This comet appeared in 1826, and 
was found to have a period of about six years and two thirds. 
On its return in 1845, ft met with a singular, and as yet unex- 



294 



ASTRONOMY. 




plained, accident, which has rendered the otherwise rather 

insignificant comet famous. In November and December of 

that year it was observed as usual, without any thing, remarka- 
ble about it; but, in 
January of the fol- 
lowing year, it was 
found to have been 
divided into two dis- 
tinct parts, so as to 
appear as two comets 
instead of one. The 
two parts were at 
first of very unequal 
brightness ; but, dur- 
ing the following 
month, the smaller 
Flg - 329> of the two increased 

in brilliancy until it equalled its companion ; it then grew 

fainter till it entirely disappeared, a month before its companion. 

The two parts were about two hundred thousand miles apart. 

Fig. 331 shows these 

two parts as they 

appeared on the 19th 

of February, and Fig. 

332 as they appeared 

on the 2 1 st of Feb- 
ruary. On its return 

in 1852, the comets 

were found still to 

be double ; but the 

two components were 

now about a million 

and a half miles 

apart. They are 



shown in Fk 



333 




as they appeared at Fig. 330. 

this time. Sometimes one of the parts appeared the brighter, 
and sometimes the other; so that it was impossible to decide 
which was really the principal comet. The two portions passed 



ASTRONOMY. 295 

out of view in September, and have not been seen since ; 
although in 1872 the position of the comet would have been 
especially favorable for observation. The comet appears to 
have become completely broken up. 




Fig. 331. 

300. The Comet of 184 J. — The great comet of 1843, a view 
of which is given in Fig. 334, was favorably situated for obser- 
vation only in southern latitudes. It was exceedingly brilliant, 




Fig. 33 2 - 

and was easily seen in full daylight, in close proximity to the 
sun. The apparent length of its tail was sixty-five degrees, 
and its real length a hundred and fifty million miles, or nearly 



296 ASTRONOMY. 

twice the distance from the earth to the sun. This comet is 
especially remarkable on account of its near approach to the 
sun. At the time of its perihelion passage the distance of 
the comet from the photosphere of the sun was less than 
one-fourteenth of the diameter of the sun. This distance was 
only one-half that of the comet of 1680 when at its perihelion. 
When at perihelion, this comet was plunging through the sun's 
outer atmosphere at the rate of one million, two hundred and 
eighty thousand miles an hour. It passed half way round the 
sun in the space of two hours, and its tail was whirled round 
through a hundred and eighty degrees in that brief time. As 




Fig. 333- 

the tail extended almost double the earth's distance from the 
sun, the end of the tail must have traversed in two hours a 
space nearly equal to the circumference of the earth's orbit, — 
a distance which the earth, moving at the rate of about twenty 
miles a second, is a whole year in passing. It is almost impos- 
sible to suppose that the matter forming this tail remained the 
same throughout this tremendous sweep. 

301. DonatVs Co7net. — The great comet of 1858, known as 
DoiiatVs comet, was one of the most magnificent of modern 
times. When at its brightest it was only about fifty million 
miles from the earth. Its tail was then more than fifty mil- 
lion miles long. Had the comet at this time been directly 
between the earth and sun, the earth must have passed through 






ASTRONOMY. 



297 



its tail; but this did not occur. The orbit of this comet was 

found to be decidedly elliptic, with a period of about two thou. 

sand years. This comet is especially celebrated on account 

of the careful telescopic observations of its nucleus and coma 

at the time of its perihelion passage. Attention has already 

been called (287) to the changes it 

underwent at that time. Its tail was 

curved, and of a curious feather-like 

form, as shown in Fig. 335. At times 

it developed lateral streamers, as shown 

in Fig. 336. Fig. 337 shows the head 

of the comet as it was seen by Bond 

of the Harvard Observatory, whose 

delineations of this comet have been 

justly celebrated. 

302. The Comet of 1861. — The great 
comet of 1 861 is remarkable for its 
great brilliancy, for its peculiar fan- 
shaped tail, and for the probable pas- 
sage of the earth through its tail. Sir 
John Herschel declared that it far ex- 
ceeded in brilliancy any comet he had 
ever seen, not excepting those of 181 1 
and 1858. Secchi found its tail to be 
a hundred and eighteen degrees in 
length, the largest but one on record. 
Fig. 338 shows this comet as it appeared 
at one time. Fig. 339 shows the posi- 
tion of the earth at E, in the tail of 
this comet, on the 30th of June, 1861. 
Fig. 340 shows the probable passage of 
the earth through the tail of the comet 
on that date. As the tail of a comet 
doubtless consists of something much 
less dense than our atmosphere, it is not surprising that no 
noticeable effect was produced upon us by the encounter, if it 
occurred. 

303. Coggia^s Comet. — This comet, which appeared in 1874, 
looked very large, because it came very near the earth. It was 




Fig. 334- 



298 



ASTRONOMY. 







Fig. 335- 
not at all brilliant. Its nucleus was carefully studied, and was 




Fig. 336. 



ASTRONOMY. 299 

found to develop a series of envelops similar to those of 
Donati's comet. Figs. 341 and 342 are two views of the head 




Fig. 337- 

of this comet. Fig. 343 shows the system of envelops that 
were developed during its perihelion passage. 



3°o 



ASTRONOMY. 



304. The Comet of June, 1881. — This comet, though far 
from being one of the largest of modern times, was still very 




Fig. 338. 

brilliant. It will ever be memorable as the first brilliant comet 
which has admitted of careful examination with the spec- 
troscope. 




Fig. 339- 

Connection between Meteors and Comets. 

305. Shooting-Stars. — On watching the heavens any 
clear night, we frequently see an appearance as of a star 



ASTRONOMY. 3OI 

shooting rapidly through a short space in the sky, and then 
suddenly disappearing. Three or four such shooting-stars 
may, on the average, be observed in the course of an hour. 
They are usually seen only a second or two • but they some- 
times move slowly, and are visible much longer. These 
stars begin to be visible at an average height of about 
seventy-five miles, and they disappear at an average height 
of about fifty miles. They are occasionally seen as high 
as a hundred and fifty miles, and continue to be visible 
till within thirty miles of the earth. Their visible paths 




Fig. 340. 

vary from ten to a hundred miles in length, though they 
are occasionally two hundred or three hundred miles long. 
Their average velocity, relatively to the earth's surface, varies 
from ten to forty-five miles a second. 

The average number of shooting-stars visible to the 
naked eye at any one place is estimated at about a thou- 
sand an hour ; and the average number large enough to 
be visible to the naked eye, that traverse the atmosphere 
daily, is estimated at over eight millions. The number of 
telescopic shooting-stars would of course be much greater. 

Occasionally, shooting-stars leave behind them a trail of 



302 ASTRONOMY. 

light which lasts for several seconds. These trails are some- 
times straight, as shown in Fig. 344, and sometimes curved, 
as in Figs. 345 and 346. They often disappear like trails 
of smoke, as shown in Fig. 347. 




Fig. 341. 

Shooting-stars are seen to move in all directions through 
the heavens. Their apparent paths are, however, generally 
inclined downward, though sometimes upward ; and after 
midnight they come in the greatest numbers from that 
quarter of the heavens toward which the earth is moving 
in its journey around the sun. 



ASTRONOMY. 



303 



306. Meteors. — Occasionally these bodies are brilliant 
enough to illuminate the whole heavens. They are then 
called meteors, although this term is equally applicable 











■fflPji^Mro^ Ptf ■■''*• 








fB^PPtv ? ' wr 


?*B!E 


ill 










1 
11 


§§j^'*? j /-*y'-:-V 








By* :\^; N I-J'- '•-";, 




■ - 




j» v*«y ,".(; Hfl 

















Fig. 342. 

to ordinary shooting-stars. Such a meteor is shown in 
Fig. 348. 

Sometimes these brilliant meteors are seen to explode, 
as shown in Fig. 349 ; and the explosion is accompanied 
with a loud detonation, like the discharge of cannon. 

Ordinary shooting-stars are not accompanied by any 



304 



ASTRONOMY. 



audible sound, though they are sometimes seen to break 
in pieces. Meteors which explode with an audible sound 
are called detonating meteors. 




Fig. 343- 

307. Aerolites. — There is no certain evidence that any 
deposit from ordinary shooting-stars ever reaches the sur- 



sfflB 



Fig- 344- 

face of the earth ; though a peculiar dust has been found 
in certain localities, which has been supposed to be of 
meteoric origin, and which has been called meteoric dust. 



ASTRONOMY. 305 

But solid bodies occasionally descend to the earth from 
beyond our atmosphere. These generally penetrate a foot 
or more into the earth, and, if picked up soon after their 
fall, are found to be warm, and sometimes even hot. These 




Fig. 345- 

bodies are called aerolites. When they have a stony appear- 
ance, and contain but little iron, they are called meteoric 
stones; when they have a metallic appearance, and are 
composed largely of iron, they are called meteoric iron. 

There are eighteen well-authenticated cases in which aero- 
lites have fallen in the United States during the last sixty 




Fig. 346. 

years, and their aggregate weight is twelve hundred and fifty 
pounds. The entire number of known aerolites the date of 
whose fall is well determined is two hundred and sixty-one. 
There are also on record seventy-four cases of which the date 



306 



ASTRONOMY. 



is more or less uncertain. There have also been found eighty- 
six masses, which, from their peculiar composition, are believed 
to be aerolites, though their fall was not seen. The weight 




Fig. 347- 

of these masses varies from a few pounds to several tons. 
The entire number of aerolites of which we have any knowl- 
edge is therefore about four hundred and twentv. 




Fig. 348. 
Aerolites are composed of the same elementary substances 
as occur in terrestrial minerals, not a single new element 
having been found in their analysis. Of the sixty or more 



ASTRONOMY. 



307 



elements now recognized by chemists, about twenty have been 
found in aerolites. 

While aerolites contain no new elements, their appearance 
is quite peculiar; and the compounds found in them are so 




Fig. 349- 

peculiar as to enable us by chemical analysis to distinguish 
an aerolite from any terrestrial substance. 

Iron ores are very abundant in nature, but iron in the 
metallic state is exceedingly rare. Now, aerolites invariably 
contain metallic iron, sometimes from ninety to ninety-six per 
cent. This iron is malleable, and may be readily worked into 




Fig. 350. 

cutting instruments. It always contains eight or ten per cent 
of nickel, together with small quantities of cobalt, copper, tin, 
and chromium. This composition has never been found in 
any terrestrial 17^ineral. Aerolites also contain, usually in 
small amount, a compound of iron, nickel, and phosphorus, 
which has never been found elsewhere. 



308 



ASTRONOMY. 



Meteorites often present the appearance of having been 
fused on the surface to a slight depth, and meteoric iron is 
found to have a peculiar crystalline structure. The external 
appearance of a piece of meteoric iron found near Lock- 
port, N.Y., is shown in Fig. 350. Fig. 351 shows the peculiar 
internal structure of meteoric iron. 

308. Meteoroids. — Astronomers now universally hold 
that shooting-stars, meteors, and aerolites are all minute 
bodies, revolving, like the comets, about the sun. They 




are moving in every possible direction through the celestial 
spaces. They may not average more than one in a million 
of cubic miles, and yet their total number exceeds all calcu- 
lation. Of the nature of the minuter bodies of this class 
nothing is certainly known. The earth is continually en- 
countering them in its journey around the sun. They are 
burned by passing through the upper regions of our atmos- 
phere, and the shooting-star is simply the light of that 
burning. These bodies, which are invisible till they plunge 
into the earth's atmosphere, are called meteoroids. 

309. Origin of the Light of Meteors. — When one of 



ASTRONOMY. 309 

these meteoroids enters our atmosphere, the resistance of 
the air arrests its motion to some extent, and so converts 
a portion of its energy of motion into that of heat. The 
heat thus developed is sufficient to raise the meteoroid and 
the air around it to incandescence, and in most cases 
either to cause the meteoroid to burn up, or to dissipate it 
as vapor. The luminous vapor thus formed constitutes the 
luminous train which occasionally accompanies a meteor, 
and often disappears as a puff of smoke. When a meteo- 
roid is large enough and refractory enough to resist the 
heat to which it is exposed, its motion is sufficiently arrested, 
on entering the lower layers of our atmosphere, to cause 
it to fall to the earth. We then have an aei-olite. A 
brilliant meteor differs from a shooting-star simply in mag- 
nitude. 

310. The Intensity of the Heat to which a Meteoroid is 
Exposed. — It has been ascertained by experiment that a body 
moving through the atmosphere at the rate of a hundred and 
twenty-five feet a second raises the temperature of the air 
immediately in front of it one degree, and that the temperature 
increases as the square of the velocity of the moving body ; 
that is to say, that, with a velocity of two hundred and fifty 
feet, the temperature in front of the body would be raised 
four degrees ; with a velocity of five hundred feet, sixteen 
degrees ; and so on. To find, therefore, the temperature to 
which a meteoroid would be exposed in passing through our 
atmosphere, we have merely to divide its velocity in feet per 
second by a hundred and twenty-five, and square the quotient. 
With a velocity of forty-four miles a second in our atmosphere, 
a meteoroid would therefore be exposed to a temperature of 
between three and four million degrees. The air acts upon 
the body as if it were raised to this intense heat. At such a 
temperature small masses of the most refractory or incom- 
bustible substances known to us would flash into vapor with 
the evolution of intense light and heat. 

If one of these meteoric bodies is large enough to pass 



310 ASTRONOMY. 






through the atmosphere and reach the earth, without being 
volatilized by the heat, we have an aerolite. As it is only a 
few seconds in making the passage, the heat has not time to 
penetrate far into its interior, but is expended in melting and 
vaporizing the outer portions. The resistance of the denser 
strata of the atmosphere to the motion of the aerolite some- 
times becomes so enormous that the body is suddenly rent to 
pieces with a loud detonation. It seems like an explosion pro- 
duced by some disruptive action within the mass ; but there 
can be little doubt that it is due to the velocity — perhaps ten, 
twenty, or thirty miles a second — with which the body strikes 
the air. 

If, on the other hand, the meteoroid is so small as to be 
burned up or volatilized in the upper regions of the atmos- 
phere, we have a common shooting-star, or a meteor of greater 
or less brilliancy. 

311. Meteoric Showers. — On ordinary nights only four 
or five shooting-stars are seen in an hour, and these move 
in every direction. Their orbits lie in all possible positions, 
and are seemingly scattered at random. Such meteors are 
called sporadic meteors. On occasional nights, shooting- 
stars are more numerous, and all move in a common direc- 
tion. Such a display is called a meteoric shower. These 
showers differ greatly in brilliancy; but during any one 
shower the meteors all appear to radiate from some one 
point in the heavens. If we mark on a celestial globe the 
apparent paths of the meteors which fall during a shower, 
or if we trace them back on the celestial sphere, we shall 
find that they all meet in the same point, as shown in Fig. 
352. This point is called the radiant point. It always 
appears in the same position, wherever the observer is situ- 
ated, and does not partake of the diurnal motion of the 
earth. As the stars move towards the west, the radiant 
point moves with them. The point in question is purely 
an effect of perspective, being the " vanishing point" of 
the parallel lines in which the meteors are actually moving. 



ASTRONOMY. 3II 

These lines are seen, not in their real direction in space, 
but as projected on the celestial sphere. If we look up- 
wards, and watch snow falling through a calm atmosphere, 
the flakes which fall directly towards us do not seem to 
move at all, while the surrounding flakes seem to diverge 
from them on all sides. So, in a meteoric shower, a 
meteor coming directly towards the observer does not seem 




Fig. 352. 

to move at all, and marks the point from which all the 
others seem to radiate. 

312. The August Meteors. — A meteoric shower of no 
great brilliancy occurs annually about the 10th of August. 
The radiant point of this shower is in the constellation Per- 
seus, and hence these meteors are often called the Perseids. 
The orbit of these meteoroids has been pretty accurately 
determined, and is shown in Fig. 353. 



312 



ASTRONOMY. 






It will be seen that the perihelion point of this orbit is 
at about the distance of the earth from the sun ; so that 

the earth encounters the mete- 
ors once a year, and this takes 
place in the month of August. 
The orbit is a very eccentric 
ellipse, reaching far beyond 
Neptune. As the meteoric dis- 
play is about equally brilliant 
every year, it seems probable 
that the meteoroids form a 
stream quite uniformly dis- 
tributed throughout the whole 
orbit. It probably takes one 
of the meteoroids about a hun- 
dred and twenty-four years to 
pass around this orbit. 

313. Th e Nov em be?' Meteors. 
— A somewhat brilliant mete- 
oric shower also occurs annu- 
ally, about the 13th of Novem- 
ber. The radiant point of 
these meteors is in the con- 
stellation Leo, and hence they 
are often called the Leonids. 
Their orbit has been deter- 
mined with great accuracy, and 
is shown in Fig. 354. While 
the November meteors are not 
usually very numerous or bright, 
a remarkably brilliant display 
Flg - 353> of them has been seen once in 

about thirty-three or thirty-four years : hence we infer, that, 
while there are some meteoroids scattered throughout the 
whole extent of the orbit, the great majority are massed in 




ASTRONOMY. 



313 



a group which traverses the orbit in a little over thirty- 
three years. A conjectural form of this condensed group is 
shown in Fig. 355. The group is so large that it takes it 
two or three years to pass the perihelion point : hence there 
may be a brilliant meteoric display two or three years in 
succession. 

The last brilliant display of these meteors was in the 
years 1866 and 1867. 



The display was visible 
in this country only a 
short time before sun- 
rise, and therefore did 
not attract general atten- 
tion. The display of 
1833 was remarkably 
brilliant in this country, 
and caused great con- 
sternation among the 
ignorant and supersti- 
tious. 

3 1 4. Connection between 
Meteors and Comets. — It 
has been found that a 
comet which appeared in 
1866, and which is desig- 
nated as 1866, I., has 
exactly the same orbit 
and period as the No- 




Fig. 354. 



vember meteors, and that another comet, known as the 1862, 
III., has the same orbit as the August meteors. It has also 
been ascertained that a third comet, 1861, I., has the same 
orbit as a stream of meteors which the earth encounters in 
April. Furthermore, it was found, in 1872, that there was a 
small stream of meteors following in the train of the lost 
comet of Biela. These various orbits of comets, and meteoric 
streams are shown in Fig. 356. The coincidence of the orbits 



314 



ASTRONOMY. 



of comets and of meteoric streams indicates that these two 
classes of bodies are very closely related. They undoubtedly 
have a common origin. The fact that there is a stream of 
meteors in the train of Biela's comet has led to the sup- 
position that comets may become gradually disintegrated into 
meteoroids. 

Physical and Chemical Constitution of Comets. 

315. Physical Constitution of Telescopic Comets. — We have 
no certain knowledge of the physical constitution of telescopic 

comets. They are usually tens 
of thousands of miles in diame- 
ter, and yet of such tenuity that 
the smallest stars can readily be 
seen through them. It would 
seem that they must shine in 
part by reflected light: yet the 
spectroscope shews that their 
spectrum is composed of bright 
bands, which would indicate that 




they 



are composed, in part at 



least, of incandescent gases. It 
is, however, difficult to conceive 
how these gases become suf- 
ficiently heated to be luminous; 
and at the same time such 
gases would reflect no sunlight. 

It seems probable that these 
comets are really made up of a 
combination of small, solid par- 
ticles in the form of minute 
meteoroids, and of gases which 
are, perhaps, rendered luminous 
by electric discharges of slight 
intensity. 

316. Physical Constitution of Large Comets. — In the case 
of large comets the nucleus is either a dense mass of solid 
matter several hundred miles in diameter, or a dense group 
of meteoroids. Professor Peirce estimated that the density 



ASTRONOMY. 



3^5 



of the nucleus is at least equal to that of iron. As such a 
comet approaches the sun, the nucleus is, to a slight extent, 
vaporized, and out of this vapor is formed the coma and the 
tail. 

That some evaporating process is going on from the nucleus 
of the comet is proved by the movements of the tail. It is 
evident that the tail cannot be an appendage carried along with 
the comet, as it seems to be. It is impossible that there 
should be any cohesion in matter of such tenuity that the 
smallest stars could be seen through a million of miles of it, 
and which is, moreover, continuallv changing its form. Then, 




Fi s- 356. 
again, as a comet is passing its perihelion, the tail appears to 
be whirled from one side of the sun to another with a rapidity 
which would tear it to pieces if the movement were real. The 
tail seems to be, not something attached to the comet, and 
carried along with it, but a stream of vapor issuing from it, 
like smoke from a chimney. The matter of which it is com- 
posed is continually streaming outwards, and continually being 
replaced by fresh vapor from the nucleus. 

The vapor, as it emanates from the nucleus, is repelled by 
the sun with a force often two or three times as great as the 
ordinary solar attraction. The most probable explanation of 
this phenomenon is, that it is a case of electrical repulsion, the 
sun and the particles of the cometary mist being similarly 



3i6 



ASTRONOMY. 



electrified. With reference to this electrical theory of the 
formation of comets' tails, Professor Peirce makes the follow- 
ing observation: "In its approach to the sun, the surface of 
the nucleus is rapidly heated : it is melted and vaporized, and 
subjected to frequent explosions. The vapor rises in its atmos- 
phere with a well-defined upper surface, which is known to 
observers as an envelop. . . . The electrification of the come- 
tary mist is analogous to that of our own thunder-clouds. Any 
portion of the coma which has received the opposite kind of 
electricity to the sun and to the repelled tail will be attracted. 
This gives a simple explanation of the negative tails which have 
been sometimes seen directed towards the sun. In cases of 
violent explosion, the whole nucleus might be broken to pieces, 

and the coma dashed 
around so as to give 
varieties of tail, and even 
multiple tails. There 
seems, indeed, to be no 
observed phenomenon of 
the tail or the coma 
which is not consistent 
with a reasonable modifi- 
cation of the theory.*' 
Professor Peirce regard- 
ed comets simply as the 



fA RTH* Q# &. 




largest of the meteoroids. 



Fig. 357- 

They appear to shine partly by reflected sunlight, and partly 
by their own proper light, which seems to be that of vapor 
rendered luminous by an electric discharge of slight intensity. 

317. Collision of a Comet and the Ea?'th. — It sometimes 
happens that the orbit of a comet intersects that of the earth, 
as is shown in Fig. 357, which shows a portion of the orbit 
of Biela's comet, with the positions of the comet and of the 
earth in 1832. Of course, were a comet and the earth both to 
reach the intersection of their orbits at the same time, a col- 
lision of the two bodies would be inevitable. With reference 
to the probable effect of such a collision, Professor Newcomb 
remarks, — 

" The question is frequently asked, What would be the 






ASTRONOMY. 



317 



effect if a comet should strike the earth ? This would depend 
upon what sort of a comet it was, and what part of the comet 
came in contact with our planet. The latter might pass through 
the tail of the largest comet without the slightest effect being 
produced ; the tail being so thin and airy that a million miles 
thickness of it looks only like gauze in the sunlight. It is not 
at all unlikely that such a thing may have happened without 
ever being noticed. A passage through a telescopic comet 
would be accompanied by a brilliant meteoric shower, probably 
a far more brilliant one than has ever been recorded. No 
more serious danger would be encountered than that arising 
from a possible fall of meteorites ; but a collision between 
the nucleus of a large comet and the earth might be a serious 




Fig. 358. 

matter. If, as Professor Peirce supposes, the nucleus is a solid 
body of metallic density, many miles in diameter, the effect 
where the comet struck would be terrific beyond conception. 
At the first contact in the upper regions of the atmosphere, the 
whole heavens would be illuminated with a resplendence 
beyond that of a thousand suns, the sky radiating a light which 
would blind every eye that beheld it, and a heat which would 
melt the hardest rocks. A few seconds of this, while the 
huge body was passing through the atmosphere, and the col- 
lision at the earth's surface would in an instant reduce every 
thing there existing to fiery vapor, and bury it miles deep in 
the solid earth. Happily, the chances of such a calamity are 
so minute that they need not cause the slightest uneasiness. 
There is hardly a possible form of death which is not a thou- 
sand times more probable than this. So small is the earth in 



3 18 ASTRONOMY. 

comparison with the celestial spaces, that if one should shut 
his eyes, and fire a gun at random in the air, the chance of 
bringing down a bird would be better than that of a comet of 
any kind striking the earth." 

318. The Chemical Constitution of Comets. — Fig. 358 shows 
the bands of the spectrum of a telescopic comet of 1873, as 
seen by two different observers. Fig. 359 shows the spectrum 
of the coma and tail of the comet of 1874; and the spectrum 
of the bright comet of 1881 showed the same three bands for 




Fig. 359- 

the coma and tail. Now, these three bands are those of cer- 
tain hydrocarbon vapors : hence it would seem that the coma 
and tails of comets are composed chiefly of such vapors (315). 

II. THE ZODIACAL LIGHT. 

319. The Genei'al Appearance of the Zodiacal Light. — 
The phenomenon known as the zodiacal light consists of a 
very faint luminosity, which may be seen rising from the 
western horizon after twilight on any clear winter or spring 
evening, also from the eastern horizon just before daybreak 
in the summer or autumn. It extends out on each side 
of the sun, and lies nearly in the plane of the ecliptic. It 
grows fainter the farther it is from the sun, and can gener- 
ally be traced to about ninety degrees from that luminary, 



ASTRONOMY. 



319 




Fig. 360. 



320 



ASTRONOMY. 



when it gradually fades away. In a very clear, tropical 
atmosphere, it has been traced all the way across the 
heavens from east to west, thus forming a complete ring. 
The general appearance of this column of light, as seen 
in the morning, in the latitude of Europe, is shown in 
Fig. 360. 

Taking all these appearances together, they indicate that 
it is due to a lens-shaped appendage surrounding the sun, 

and extending a little 
beyond the earth's 
orbit. It lies nearly 
in the plane of the 
ecliptic ; but its exact 
position is not easily 
determined. Fig. 

361 shows the gen- 
eral form and posi- 
tion of this solar 
appendage, as seen 
in the west. 



320. The Visibility 
of the Zodiacal Light. 
— The reason why the 
zodiacal light is more 
favorably seen in the 
evening during the 
winter and spring than 




Fig. 361. 

in the summer and fall is evident from Fig. 362, which shows 
the position of the ecliptic and the zodiacal light with reference 
to the western horizon at the time of sunset in March and in 
September. It will be seen that in September the axis of the 
light forms a small angle with the horizon, so that the phenom- 
enon is visible only a short time after sunset and low down 
where it is difficult to distinguish it from the glimmer of the twi- 
light; while in March, its axis being nearly perpendicular to the 
horizon, the light may be observed for some hours after sunset 



ASTRONOMY. 



321 




and well up in the sky. Fig. 363 gives the position of the 

ecliptic and of the zodiacal light with reference to the eastern 

horizon at the time of 

sunrise, and shows why 

the zodiacal light is 

seen to better advan- 
tage in the morning 

during the summer and 

fall than during the 

winter and spring. It 

will be observed that 

here the angle made by 

the axis of the light 

with the horizon is small 

in March, while it is 

large in September; the 

conditions represented 

in the preceding figure 

being thus reversed. Fig. 362. 

321. Nature of the Zodiacal Light. — Various attempts have 

been made to explain 
the phenomena of the 
zodiacal light; but the 
most probable theory 
is, that it is due to 
an immense number of 
meteors which are re- 
volving around the sun, 
and which lie mostly 
within the earth's orbit. 
Each of these meteors 
reflects a sensible por- 
tion of sunlight, but is 
far too small to be sep- 
arately visible. All of 
these meteors together 
would, by their com- 
bined reflection, produce a kind of pale, diffused light. 




Fig. 3 6 3- 



III. 

THE STELLAR UNIVERSE. 



I. GENERAL ASPECT OF THE HEAVENS. 

322. The MagJiitude of the Stars. — The stars that are 
visible to the naked eye are divided into six classes, accord- 
ing to their brightness. The brightest stars are called stars 
of the first magnitude ; the next brightest, those of the 
second magnitude ; and so on to the sixth magnitude. The 
last magnitude includes the faintest stars that are visible to 
the naked eye on the most favorable night. Stars which 
are fainter than those of the sixth magnitude can be seen 
only with the telescope, and are called telescopic stars. 
Telescopic stars are also divided into magnitudes ; the divis- 
ion extending to the sixteenth magnitude, or the faintest 
stars that can be seen with the most powerful telescopes. 

The classification of stars according to magnitudes has 
reference only to their brightness, and not at all to their 
actual size. A sixth magnitude star may actually be larger 
than a first magnitude star ; its want of brilliancy being due 
to its greater distance, or to its inferior luminosity, or to 
both of these causes. 

None of the stars present any sensible disk, even in the 
most powerful telescope : they all appear as mere points of 
light. The larger the telescope, the greater is its power 
of revealing faint stars ; not because it makes these stars 
appear larger, but because of its greater light-gathering 
322 



ASTRONOMY. 323 

power. This power increases with the size of the object- 
glass of the telescope, which plays the part of a gigantic 
pupil of the eye. 

The classification of the stars into magnitudes is not 
made in accordance with any very accurate estimate of 
their brightness. The stars which are classed together in 
the same magnitude are far from being equally bright. 

The stars of each lower magnitude are about two-fifths as 
bright as those of the magnitude above. The ratio of diminu- 
tion is about a third from the higher magnitude down to the 
fifth. Were the ratio two-fifths exact, it would take about 

2-J stars of the 2d magnitude to make one of the 1st. 

6 stars of the 3d magnitude to make one of the 1st. 

16 stars of the 4th magnitude to make one of the 1st. 

40 stars of the 5th magnitude to make one of the 1st. 

100 stars of the 6th magnitude to make one of the 1st. 

10,000 stars of the nth magnitude to make one of the 1st. 

1,000,000 stars of the 16th magnitude to make one of the 1st. 

323. The Number of the Stars. — The total number 
of stars in the celestial sphere visible to the average naked 
eye is estimated, in round numbers, at five 
thousand ; but the number varies much with 
the perfection and the training of the eye 
and with the atmospheric conditions. For 
every star visible to the naked eye, there are 
thousands too minute to be seen without Flg - 364 - 

telescopic aid. Fig. 364 shows a portion of the constella- 
tion of the Twins as seen with the naked eye ; and Fig. 365 
shows the same region as seen in a powerful telescope. 

Struve has estimated that the total number of stars visible 
with Herschel's twenty-foot telescope was about twenty 
million. The number that can be seen with the great tele- 
scopes of modern times has not been carefully estimated, 
but is probably somewhere between thirty million and fifty 
million. 




324 



ASTRONOMY. 



The number of stars between the north pole and the circle 
thirty-five degrees south of the equator is about as follows : — 

Of the ist magnitude about 14 stars. 

Of the 2d magnitude about 48 stars. 

Of the 3d magnitude about 152 stars. 

Of the 4th magnitude about 313 stars. 

Of the 5th magnitude about 854 stars. 

Of the 6th magnitude about 2010 stars. 

Total visible to naked eye 3391 stars. 




Fig. 365. 

The number of stars of the several magnitudes is approxi- 
mately in inverse proportion to that of their brightness, the 
ratio being a little greater in the higher magnitudes, and proba- 
bly a little less in the lower ones. 



ASTRONOMY. 



325 



324. The Division of the Stars into Constellations. — 
A glance at the heavens is sufficient to show that the stars 
are not distributed uniformly over the sky. The larger ones 
especially are collected into more or less irregular groups. 
The larger groups are called constellations. At a very early 
period a mythological figure was allotted to each constella- 
tion ; and these figures were drawn in such a way as to 
include the principal stars of each constellation. The 
heavens thus became covered, as it were, with immense 
hieroglyphics. 

There is no historic record of the time when these figures 
were formed, or of the principle in accordance with which they 
were constructed. It is probable that the imagination of the 
earlier peoples may, in many instances, have discovered some 
fanciful resemblance in the configuration of the stars to the 
forms depicted. The names are still retained, although the 
figures no longer serve any astronomical purpose. The con- 
stellation Hercules, for instance, no longer represents the figure 
of a man among the stars, but a certain portion of the heavens 
within which the ancients placed that figure. In star- maps 
intended for school and popular use it is still customary to 
give these figures ; but they are not generally found on maps 
designed for astronomers. 

325. The Naming of the Stars. — The brighter stars have 
all proper names, as Sirii/s, Proeyon, Arcturus, Capella, 
Aldebaran, etc. This method of designating the stars was 
adopted by the Arabs. Most of these names have dropped 
entirely out of astronomical use, though many are popularly 
retained. The brighter stars are now generally designated 
by the letters of the Greek alphabet, — alpha, beta, gamma, 
etc., — to which is appended the genitive of the name of 
the constellation, the first letter of the alphabet being used 
for the brightest star, the second for the next brightest, and 
so on. Thus Aldebaran would be designated as Alpha 
Tauri. In speaking of the stars of any one constellation, 



326 ASTRONOMY. 

we simply designate them by the letters of the Greek alpha 
bet, without the addition of the name of the constellation, 
which answers to a person's surname, while the Greek letter 
answers to his Christian name. The names of the seven 
stars of the "Dipper" are given in Fig. 366. When the 
letters of the Greek alphabet are exhausted, those of the 
Roman alphabet are employed. The fainter stars in a 
constellation are usually designated by some system of 
numbers. 

326. The Milky -Way, or Galaxy. — The Milky- Way is 



•' 




• 








1 

• 1 

• 


& '^^-^^^B^^^j^BB 







Fig. 366. 

a faint luminous band, of irregular outline, which surrounds 
the heavens with a great circle, as shown in Fig. 367. 
Through a considerable portion of its course it is divided 
into two branches, and there are various vacant spaces at 
different points in this band ; but at only one point in the 
southern hemisphere is it entirely interrupted. 

The telescope shows that the Galaxy arises from the 
light of countless stars too minute to be separately visible 
with the naked eye. The telescopic stars, instead of being 
uniformly distributed over the celestial sphere, are mostly 



ASTRONOMY. 



327 




Fig. 367. 



328 



ASTRONOMY. 



condensed in the region of the Galaxy. They are fewest 
in the regions most distant from this belt, and become 
thicker as we approach it. The greater the telescopic 
power, the more marked is the condensation. With the 
naked eye the condensation is hardly noticeable ; but with 

the aid of a very small tele- 
scope, we see a decided thicken- 
ing of the stars in and near the 
Galaxy, while the most power- 
ful telescopes show that a large 
majority of the stars lie actually 
in the Galaxy. If all the stars 
visible with a twelve-inch tele- 
scope were blotted out, we 
should find that the greater part 
of those remaining were in the 
Galaxy. 

The increase in the number 
of the stars of all magnitudes as 
we approach the plane of the 
Milky-Way is shown in Fig. 368. 
The curve acb shows by its 
height the distribution of the 
stars above the ninth magnitude, 
and the curve A CB those of 
all magnitudes. 

327. Star - Clusters. — Besides 
this gradual and regular con- 
densation towards the Galaxy, 
occasional aggregations of stars 
Some of these are visible to the 
separate stars, like the " Seven 




Fig. 368. 



into clusters may be seen, 
naked eye, sometimes as 
Stars," or Pleiades, but more commonly as patches of dif- 
fused light, the stars being too small to be seen separately. 
The number visible in powerful telescopes is, however, much 



ASTRONOMY. 



329 




Fig. 369. 



330 ASTRONOMY. 






greater. Sometimes hundreds or even thousands of stars 
are visible in the field of view at once, and sometimes the 
number is so great that they cannot be counted. 

328. Nebula. — Another class of objects which are found 
in the celestial spaces are irregular masses of soft, cloudy 
light, known as nebulce. Many objects which look like 
nebulae in small telescopes are shown by more powerful 
instruments to be really star-clusters. But many of these 
objects are not composed of stars at all, being immense 
masses of gaseous matter. 

The general distribution of nebulae is the reverse of that 
of the stars. Nebulae are thickest where stars are thinnest. 
While stars are most numerous in the region of the Milky- 
Way, nebulae are most abundant about the poles of the 
Milky- Way. This condensation of nebulae about the poles 
of the Milky- Way is shown in Figs. 367 and 369, in which 
the points represent, not stars, but nebulae. 

II. THE STARS. 
The Constellations. 

329. The Great Bear. — The Great Bear, or Ursa Major, 
is one of the circumpolar constellations (4), and contains one 
of the most familiar asteris/ns, or groups of stars, in our 
sky; namely, the Great Dipper, or Charleses Wain. The 
positions and names of the seven prominent stars_in it are 
shown in Fig. 370. The two stars Alpha and Beta are called 
the Pointers. This asterism is sometimes called the Butchers 
Cleaver. The whole constellation is shown in Fig. 371. A 
rather faint star marks the nose of the bear, and three equi- 
distant pairs of faint stars mark his feet. 

330. The Little Bear, Draco, and Cassiopeia. — These are 
all circumpolar constellations. The most important star of the 
Little Bear, or Ursa Minor, is Polaris, or the Pole Star. This 
star may be found by drawing a line from Beta to Alpha of the 
Dipper, and prolonging it as shown in Fig. 372. This explains 
why these stars are called the Pointers. The Pole Star, with 



ASTRONOMY. 33 1 

the six other chief stars of the Little Bear, form an asterism 




Fig. 370. 
called the Little Dipper. These six stars are joined with 
Polaris by a dotted line in Fig. 372. 




Fig. 37 1 - 

The stars in a serpentine line between the two Dippers are 
the chief stars of Draco, or the Dragon j the trapezium mark- 



332 



ASTRONOMY. 



ing its head. Fig. 373 shows the constellations of Ursa Minor 
and Draco as usually figured. 




Fig. 372. 

To find Cassiopeia^ draw a line from Delta of the Dipper to 



ASTRONOMY. 



333 




Fig. 373- 

Polaris, and prolong it about an equal distance beyond, as 




Fig- 374- 

shown in Fig. 372. This line will pass near Alpha of Cassio- 



334 



ASTRONOMY. 



peia. The five principal stars of this constellation form an 
irregular W, opening towards the pole. Between Cassiopeia 
and Draco are five rather faint stars, which form an irregular 
K. These are the principal stars of the constellation Cepheus. 
These two constellations are shown in Fig. 374. 

331. The Lion, Berenice's Hair, and the Hunting-Dogs. — 
A line drawn from Alpha to Beta of the Dipper, and prolonged 
as shown in Fig. 375, will pass between the two stars Denebola 
and Regulus of Leo, or the Lion. Regulus forms a sickle with 




Fig. 375- 

several other faint stars, and marks the heart of the lion. 
Denebola is at the apex of a right-angled triangle, which it 
forms with two other stars, and marks the end of the lion's 
tail. This constellation is visible in the evening from February 
to July, and is figured in Fig. 376. 

In a straight line between Denebola and Eta, at the end of 
the Great Bear's tail, are, at about equal distances, the two 
small constellations of Coma Berenices, or Berenice's Hair, 
and Canes Venatici, or the Hun ting-Dogs. These are shown 
in Fig. 377. The dogs are represented as pursuing the bear, 
urged on by the huntsman Bootes. 



ASTRONOMY. 



335 




Fig. 376. 
332. Bootes, Hercules, and the A T orthern Crown. — Arc 




Fig- 377- 



turns, the principal star of Bootes, may be found by drawing 



336 ASTRONOMY. 






a line from Zeta to Eta of the Dipper, and then prolonging 
it with a slight bend, as shown in Fig. 378, Arcturus and 
Polaris form a large isosceles triangle with a first-magnitude 
star called Vega. This triangle encloses at one corner the 
principal stars of Bootes, and the head of the Dragon near 
the opposite side. The side running from Arcturus to Vega 
passes through Corona Borealis, or the Northern Crown, and 




Fig. 378. 
the body of Hercules, which is marked by a quadrilateral of 
four stars. 

Bootes, who is often represented as a husbandman. Corona 
Borealis, and Hercules, are delineated in Fig. 37.9. These 
constellations are visible in the evening from May to September. 

333. The Lyre, the Swan, the Eagle, and the Dolphin. — 
Altair, the principal star of Aquila, or the Eagle, lies on the 
opposite side of the Milky- Way from Vega. Altair is a first- 



ASTRONOMY. 337 

magnitude star, and has a faint star on each side of it. as 



Fig. 379- 

shown in Fig, 380, Vega, also of the first magnitude, is the 



338 ASTRONOMY. 

principal star of Lyra, or the Lyre, Between these two stars, 
and a little farther to the north, are several stars arranged in 
the form of an immense cross. The bright star at the head 
of this cross is called Deneb. The cross lies in the Milky- Way, 
and contains the chief stars of the constellation Cygnus, or 
the Swan. A little to the north of Altair are four stars in the 
form of a diamond. This asterism is popularly known as Job^s 




Fig. 380. 

Coffin. These four stars are the chief stars of Delphinus, or 
the Dolphin. These four constellations are shown together in 
Fig. 381. The Swan is visible from June to December, in the 
evening. 

334. Virgo. — A line drawn from Alpha to Gamma of the 
Dipper, and prolonged with a slight bend at Gamma, will reach 
to a first-magnitude star called Spica (Fig. 382). This is the 
chief star of the constellation Virgo, or the Virgin, and forms 
a large isosceles triangle with A returns and Denebola. 



ASTRONOMY. 339 

Virgo is represented in Fig. 383. To the right of this con- 




Fig. 381. 
stellation, as shown in the figure, there are four stars which 




Fig. 382. 

form a trapezium, and mark the constellation Corvus, or the 



340 ASTRONOMY. 

Crow. This bird is represented as standing on the body of 
Hydra, or the Water-Snake. Virgo is visible in the evening, 
from April to August. 

335. The Twins. — A line drawn from Delta to Beta of the 
Dipper, and prolonged as shown in Fig. 384, passes between 
two bright stars called Castor and Pollux. The latter of these 
is usually reckoned as a first-magnitude star. These are the 




principal stars of the constellation Gemini, or the Twins, 
which is shown in Fig. 385. The constellation Canis Minor, 
or the Little Dog, is shown in the lower part of the figure. 
There are two conspicuous stars in this constellation, the 
brightest of which is of the first magnitude, and called Procyon. 
The region to which we have now been brought is the 
richest of the northern sky, containing no less than seven first- 
magnitude stars. These are Sirius, Pxoeyon, Pollux. Capella, 
Aldebaran, Betelgeuse. and Rigel. They are shown in Fig. 386. 



ASTRONOMY. 



H* 




Fig. 384. 
Belelgeuse and Rigel are in the constellation Orion, being 







>4CP? 



*4 ^ 



\/^ 




^' /-» 


v- 






or? 1 


/ 


) ■/ 


< VI 


/\ 




I ( 


X^\ 


C\> 




VS 









Fig. 385- 



342 



ASTRONOMY. 



about equally distant to the north and south from the three 
stars forming the belt of Orion. Betelgeuse is a red star. 
Sirius is the brightest star in the heavens, and belongs to the 
constellation Cam's Major, or the Great Dog. It lies to the 
east of the belt of Orion. A Idebaran 'lies at about the same 
distance to the west of the belt. It is a red star, and belongs 



Capellii 


• 




H Si! 


• 


.. • 




• 


o 




i 


. ;•< 


, 


• . 


Uiiebaran, 




' V 






1 ,'*' 






'• 


• 


1 ' 


• 


• 


• 
• 


• 




• 


91 






• 


1 

• K)?e!,f 

4 


• Castor, 










Betel 


geuse. 


•• •• 


• 
Pollux, . 








kMmaKB 
kFBBBH 








• Prooyon. 





















Sirins, 

• 



















Fig. 386. 

to the constellation Taurus, or the Bull. Capella is in the 
constellation Auriga, or the Wagoner. These stars are visible 
in the evening, from about December to April. 

336. Orion and his Dogs, and. Taurus. — Orion and his 
Dogs are shown in Fig. 387, and Orion and Taurus in Fig. 388. 
Aldebaran marks one of the eyes of the bull, and is often called 
the Bull's Eye. The irregular V in the face of the bull is 
called the Hyades, and the cluster on the shoulder the Pleiades. 



ASTRONOMY. 



343 



337. The Wagoner. — The constellation Auriga, or the 
Wagoner (sometimes called the Charioteer), is shown in Fig. 
389. Capella marks the Goat, which he is represented as 
carrying on his back, and the little right-angled triangle of 
stars near it the Kids. The five chief stars of this constella- 
tion form a large, irregular pentagon. Gamma of Auriga is 




Fig. 387. 

also Beta of Taurus, and marks one of the horns of the Bull. 
338. Pegasus, Andromeda, and Perseus. — A line drawn 
from Polaris near to Beta of Cassiopeia will lead to a bright 
second-magnitude star at one corner of a large square (Fig. 390). 
Alpha belongs both to the Square of Pegasus and to Androm- 
eda. Beta and Gamma, which are connected with Alpha in 
the figure by a dotted line, also belong to Andromeda. Algol, 



344 



ASTRONOMY. 




Fig. 388. 
which forms, with the last-named stars and with the Square of 




Fig. 389. 



ASTRONOMY. 



345 




Fig-. 390. 

Pegasus, an asterism similar in configuration to the Great 




Fig. 39 1 - 




Fig. 39?. 



Dipper, belongs to Perseus. Algenib, which is reached by 
bending the line at Gamma in the opposite direction, is the 
principal star of Perseus. 




Pegasus is shown in Fig. 391, and Andromeda in Fig. 392= 
Cetus, the Whale, or the Sea Monster, shown in Fig. 393. 
belongs to the same mythological group of constellations. 



ASTRONOMY. 



347 



339. Scorpio, Sagittarius, and Ophiuchus. — During the 
summer months a brilliant constellation is visible, called Scor- 
pio, or the Scorpion. The configuration of the chief stars of 
this constellation is shown in Fig. 394. They bear some 
resemblance to a boy's kite. The brightest star is of the first 
magnitude, and called An tares (from anti, instead of, and Ares, 
the Greek name of Mars), because it rivals Mars in redness. 
The stars in the tail of the Scorpion are visible in our latitude 
only under very favorable circumstances. This constellation 




Fig. 394- 

is shown in Fig. 395, together with Sagittarius and Ophiuchus. 
Sagittarius, or the Archer, is to the east of Scorpio. It con- 
tains no bright stars, but is easily recognized from the fact 
that five of its principal stars form the outline of an inverted 
dipper, which, from the fact of its being partly in the Milky- 
Way, is often called the Milk Dipper. 

Ophiuchus, or the Serpent- Bearer, is a large constellation, 
filling all the space between the head of Hercules and Scor- 
pio. It is difficult to trace, since it contains no very brilliant 
stars. This constellation and Libra, or the Balances, which is 



348 



ASTRONOMY. 




Fig. 395- 



ASTRONOMY 



540 



the zodiacal constellation to the west of Scorpio, are shown 
in Fig. 396. 




Fig. 396.' 

340. Capricornus, Aquarius, and the Southern Fish. — The 




Fig. 397- 

two zodiacal constellations to the east of Sagittarius are Capri- 



350 ASTRONOMY. 

cornus and Aquarius. Capricornus contains three pairs of 
small stars, which mark the head, the tail, and the knees of the 
animal. 

Aquarius is marked by no conspicuous stars. An irregular 
line of minute stars marks the course of the stream of water 
which flows from the Wafer-Bearer's Urn into the mouth of the 
Southern Fish. This mouth is marked by the first-magnitude 
star Fo7nalhaut. These constellations are shown in Fig. 397. 

341. Pisces and Aries. — The remaining zodiacal constella- 
tions are Pisces, or the Fishes, Aries, or the Ram (Fig. 398), 
and Cancer, or the Crab. 




Fig. 398. 
The Fishes lie under Pegasus and Andromeda, but contain 
no bright stars. Aries (between Pisces and Taurus) is marked 
by a pair of stars on the head, — one of the second, and one 
of the third magnitude. Cancer (between Leo and Gemini) has 
no bright stars, but contains a remarkable cluster of small stars 
called Prozsepe, or the Beehive. 

Clusters. 

342. The Hyades. — The Hyades are a very open cluster in 
the face of Taurus (334). The three brightest stars of this 
cluster form a letter V, the point of the V being on the nose, 
and the open ends at the eyes. This cluster is shown in Fig. 



ASTRONOMY. 



351 




399. The name, according to the most probable etymology, 

means rainy ; and 

they are said to have 

been so called be- 
cause their rising was 

associated with wet 

weather. They were 

usually considered 

the daughters of 

Atlas, and sisters of 

the Pleiades, though 

sometimes referred 

to as the nurses of 

Bacchus. 

343. The Pleiades. 

— The Pleiades con- 
stitute a celebrated Fi s- 399- 

group of stars, or a miniature constellation, on the shoulder 

of Tanrits. Hesiod 
mentions them as 
" the seven virgins 
of Atlas born," and 
Milton calls them 
" the seven Atlantic 
sisters." They are 
referred to in the 
Book of Job. The 
Spaniards term them 
"the little nanny- 
goats ; " and they are 
sometimes called "the 
hen and chickens." 

Usually only six 
stars in this cluster 
can be seen with the 
naked eye, and this 
fact has given rise 
Fi s- 400. t0 the legend of the 

"lost Pleiad." On a clear, moonless night, however, a good 




352 ASTRONOMY. 






eye can discern seven or eight stars, and some observers have 
distinguished as many as eleven. Fig. 400 shows the Pleiades 




Fig. 401. 

as they appear to the naked eye under the most favorable 
circumstances. Fig. 401 shows this cluster as it appears in 



ASTRONOMY, 



3 5 3 



a powerful telescope. With such an instrument more than five 
hundred stars are visible. 

344. Cluster in the Sword-handle of Perseus. — This is a 
somewhat dense double clus- 
ter. It is visible to the naked 
eve, appearing as a hazy star. 
A line drawn from Algenib. or 
Alpha of Perseus (338), to Delta 
of Cassiopeia (330), will pass 
through this cluster at about 
two-thirds the distance from 
the former. This double clus- 
ter is one of the most brilliant 
objects in the heavens, with a 
telescope of moderate power. 

345. Cluster of Hercules. — Fig. 402. 

The celebrated globular cluster of Hercules can be seen only 
with a telescope of considerable power, and to resolve it into 





Fig. 403. 



distinct stars (as shown in Fig. 402) requires an instrument of 
the very highest class. 



354 




346. Other C luster s.- 




Fig. 405. 

magnitude being immensely numerous, 
densation of light at the centre. 



Fig. 403 shows a magnificent globular 
cluster in the con- 
stellation Aquarius. 
Herschel describes it 
as appearing like a 
heap of sand, being 
composed of thou- 
sands of stars of the 
fifteenth magnitude. 

Fig. 404 shows a 
cluster in the con- 
stellation Toucan, 
which Sir John Her- 
schel describes as a 
most glorious globu- 
lar cluster, the stars 
of the fourteenth 
There is a marked con- 



ASTRONOMY. 



355 



Fig. 405 shows a cluster in the Centaur, which, according 
the same astrono- 
ner, is beyond corn- 
prison the richest and 
argest object of the 
and in the heavens, 
:he stars in it being 
iterally innumerable. 
Fig. 406 shows a clus- 
:er in Scorpio, remarka- 
ble for the peculiar 
irrangement of its com- 
ponent stars. 

Star clusters are 
especially abundant in 
the region of the Milky- 
Way, the law of their Fig. 406. 
distribution being the reverse of that of the nebulas. 

Double and Multiple Stars. 

347. Double Stars, — The telescope shows that many 
stars which appear single to the naked eye are really double, 
or composed of a pair of stars lying side by side. There 
are several pairs of stars in the heavens which lie so near 





Fig. 407. Fig. 408. 

together that they almost seem to touch when seen with 
the naked eye. 

Pairs of stars are not considered double unless the com- 
ponents are so near together that they both appear in the 



356 



ASTRONOMY, 



field of view when examined with a telescope. In the 
majority of the pairs classed as double stars the distance 
between the components ranges from half a second to 
fifteen seconds. 

Epsilon Lyrcz is a good 
example of a pair of 
stars that can barely be 
separated with a good 
eye. Figs. 407 and 408 
show this pair as it ap- 
pears in telescopes mag- 
nifying respectively four 
and fifteen times; and 
Fig. 409 shows it as seen 
in a more powerful tele- 
scope, in which each of F,g - 4 ° 9 ' 
the two components of the pair is seen to be a truly double 
star. 

348. Multiple Stars. — When a star is resolved into 
more than two components by a telescope, it is called a 
multiple star. Fig. 410 shows a triple star in Pegasus. 






Fig. 410. 



Fig. 411. 



Fig. 411 shows a quadruple star in Taicrus. Fig. 412 
shows a sextuple star, and Fig. 413 a septuple star. Fig. 
414 shows the celebrated septuple star in Orion, called 
Theta Orionis, or the trapezium of Orion. 



ASTRONOMY. 



357 



349. Optically Double and Multiple Stars. — Two or 
more stars which are really very distant from each other, 
and which have no physical connection whatever, may 
appear to be near together, because they happen to lie in 
the same direction, one behind the other. Such accidental 
combinations are called optically double or multiple stars. 





350. Physically Double and Multiple Stars. — In the 
majority of cases the components of double and multiple 
stars are in reality comparatively near together, and are 
bound together by gravity into a physical system. Such 
combinations are called physi- 
cally double and multiple stars. 
The components of these sys- 
tems all revolve around their 
common centre of gravity. In 
many instances their orbits and 
periods of revolution have been 
ascertained by observation and ¥{ s- 4*4- 
calculation. Fig. 415 shows the orbit of one of the com- 
ponents of a double star in the constellation Hercules. 

351. Colors of Double and Multiple Stars. — The com- 
ponents of double and multiple stars are often highly col- 
ored, and frequently the components of the same system 
are of different colors. Sometimes one star of a binary 
system is white, and the other red ; and sometimes a white 




358 ASTRONOMY. 

star is combined with a blue one. Other colors found ir 
combination in these systems are red and blue, orange and 
green, blue and green, yellow and blue, yellow and red, etc. 
If these double and multiple stars are accompanied b\ 




Fig. 415. 

planets, these planets will sometimes have two or more suns 
in the sky at once. On alternate days they may have suns 
of different colors, and perhaps on the same day two 
suns of different colors. The effect of these changing 
colored lights on the landscape must be very remarkable. 

New and Variable Stars. 

352. Variable Stars. — There are many stars which 
undergo changes of brilliancy, sometimes slight, but occa- 
sionally very marked. These changes are in some cases 
apparently irregular, and in others periodic. All such stars 
are said to be variable, though the term is applied espe- 
cially to those stars whose variability is periodic. 

353- Algol. — Algol, a star of Perseus, whose position is 



ASTRONOMY. 359 

shown in Fig. 416, is a remarkable variable star of a short 
period. Usually it shines as a faint second-magnitude star ; 
but at intervals of a little less than three days it fades to 
the fourth magnitude for a few hours, and then regains its 
former brightness. These changes were first noticed some 
two centuries ago, but it was not till 1782 that they were 
accurately observed. The period is now known to be two 
days, twenty hours, forty-nine minutes. It takes about four 
hours and a half to fade away, and four hours more to 
recover its brilliancy. Near the beginning and end of the 
variations, the change is very 
slow, so that there are not more 
than five or six hours during 
which an ordinary observer 
would see that the star was 
less bright than usual. 

This variation of light was at 
first explained by supposing that 
a large dark planet was revolv- 
ing round Algol, and passed 
over its face at every revolution, 
thus cutting off a portion of its 
light ; but there are small irregu- 
larities in the variation, which 
this theory does not account for. Fig. 416. 

354. Mira. — Another remarkable variable star is Omicron 
Ceti, or Mira (that is, the wonderful star) . It is generally 
invisible to the naked eye ; but at intervals of about eleven 
months it shines forth as a star of the second or third 
magnitude. It is about forty days from the time it becomes 
visible until it attains its greatest brightness, and is then 
about two months in fading to invisibility ; so that its 
increase of brilliancy is more rapid than its waning. Its 
period is quite irregular, ranging from ten to twelve months ; 
so that the times of its appearance cannot be predicted 




360 ASTRONOMY. 

with certainty. Its maximum brightness is also variable, 
being sometimes of the second magnitude, and at others 
only of the third or fourth. 

355. Eta Argus. — Perhaps the most extraordinary varia- 
ble star in the heavens is Eta Argus, in the constellation 
Arga, or the Ship, in the southern hemisphere (Fig. 417). 
The first careful observations of its variability were made 
by Sir John Herschel while at the Cape of Good Hope. 
He says, "It was on the 16th of December, 1837, that, 
resuming the photometrical comparisons, my astonishment 
was excited by the appearance of a new candidate for dis- 
tinction among the very brightest stars of the first magni- 
tude in a part of the heavens 
where, being perfectly familiar 
with it, I was certain that no 
such brilliant object had before 
been seen. After a momentary 
hesitation, the natural conse- 
quence of a phenomenon so 
utterly unexpected, and refer- 
ring to a map for its configura- 
tion with other conspicuous 
Fig. 417. -tars in the neighborhood, I 

became satisfied of its identity with my old acquaintance, 
Eta Argus. Its light was, however, nearly tripled. While 
yet low, it equalled Rigel. and, when it attained some 
altitude, was decidedly greater." It continued to increase 
until Jan. 2, 1838, then faded a little till April following, 
though it was still as bright as Aldebaran. In 1842 and 
1843 ft blazed up brighter than ever, and in March of the 
latter year was second only to Sirius. During the twenty- 
five years following it slowly but steadily diminished. In 
1867 it was barely visible to the naked eye; and the next 
year it vanished entirely from the unassisted view, and has 
not yet begun to recover its brightness. The curve in 




ASTRONOMY. 36 1 

Fig. 418 shows the change in brightness of this remarkable 
star. The numbers at the bottom show the years of the 
century, and those at the side the brightness of the star. 

356. New Stars, — In several cases stars have suddenly 
appeared, and even become very brilliant ; then, after a 
longer or shorter time, they have faded away and disap- 
peared. Such stars are called new or temporary stars. 
For a time it was supposed that -such stars were actually 
new. They are now, however, classified by astronomers 
among the variable stars, their changes being of a very 
irregular and fitful character. There is scarcely a doubt 
that they were all in the heavens as very small stars before 













IbE !!■■■■■ ISCSHH 






mi mwmm iinihiI 






mi mmmmmwm 






n HM 












■ 



they blazed forth in so extraordinary a manner, and that 
they are in the same places still. There is a wide difference 
between these irregular variations, or the breaking-forth of 
light on a single occasion in the course of centuries, and 
the regular and periodic changes in the case of a star like 
Algol ; but a long series of careful observation has resulted 
in the discovery of stars of nearly every degree of irregu- 
larity between these two extremes. Some of them change 
gradually from one magnitude to another, in the course of 
years, without seeming to follow any law whatever ; while 
in others some slight tendency to regularity can be traced. 
Eta Argus may be regarded as a connecting link between 
new and variable stars. 

357. Tycho Brake's Star. — An apparently new star 



362 



ASTRONOMY. 



suddenly appeared in Cassiopeia in 1572. It was first seen 
by Tycho Brahe, and is therefore associated with his name. 
Its position in the constellation is shown in Fig. 419. It 
was first seen on Nov. 11, when it had already attained the 
first magnitude. It became rapidly brighter, soon rivalling 
Venus in splendor, so that good eyes could discern it in 
full daylight. In December it began to wane, and gradu- 
ally faded until the following May, when it disappeared 
entirely. 

A star showed itself in the same part of the heavens in 
945 and in 1264. If these were three appearances of the 

same star, it must 
be reckoned as a 
periodic star with 
a period of a little 
more than three 
hundred years. 

358. Kepler's 
Star. — In 1604 a 
new star was seen 
in the constellation 
Ophiuchus. It was 
first noticed in 
October of that 
In the following 
winter it began to fade, but remained visible during the 
whole year 1605. Early in 1606 it disappeared entirely. 
A very full history of this star was written by Kepler. 

One of the most remarkable things about this star was 
its brilliant scintillation. According to Kepler, it displayed 
all the colors of the rainbow, or of a diamond cut with 
multiple facets, and exposed to the rays of the sun. It is 
thought that this star also appeared in 393. 798, and 1203; 
if so, it is a variable star with a period of a little over four 
hundred vears. 




Fig. 419. 



year, when it was of the first magnitude 



ASTRONOMY. 363 

359. New Star of 1866. — The most striking case of 
this kind in recent times was in May, 1866, when a star 
of the second magnitude suddenly appeared in Corona 
Borealis, On the nth and 12th of that month it was 
observed independently by at least five observers in Europe 
and America. The fact that none of these new stars were 
noticed until they had nearly or quite attained their greatest 
brilliancy renders it probable that they all blazed up very 
suddenly. 

360. Cause of the Variability of Stars. — The changes in 
the brightness of variable and temporary stars are probably 
due to operations similar to those which produce the spots and 
prominences in our sun. We have seen (188) that the fre- 
quency of solar spots shows a period of eleven years, during 
one portion of which there are few or no spots to be seen, while 
during another portion they are numerous. If an observer so 
far away as to see our sun like a star could from time to time 
measure its light exactly, he would find it to be a variable star 
with a period of eleven years, the light being least when we 
see most spots, and greatest when few are visible. The varia- 
tion would be slight, but it would nevertheless exist. Now, 
we have reason to believe that the physical constitution of the 
sun and the stars is of the same general nature. It is there- 
fore probable, that, if we could get a nearer view of the stars, 
we should see spots on their disks as we do on the sun. It 
is also likely that the varying physical constitution of the stars 
might give rise to great differences in the number and size of 
the spots ; so that the light of some of these suns might van- 
to a far greater degree than that of our own sun does. If the 
variations had a regular period, as in the case of our sun. the 
appearances to a distant observer would be precisely what we 
see in the case of a periodic variable star. 

The spectrum of the new star of 1866 was found to be a 
continuous one. crossed by bright lines, which were apparently 
due to glowing hydrogen. The continuous spectrum was also 
crossed by dark lines, indicating that the light had passed 
through an atmosphere of comparatively cool gas. Mr. Huggins 



364 ASTRONOMY. 

infers from this that there was a sudden and extraordinary out- 
burst of hydrogen gas from the star, which by its own light, 
as well as by heating up the whole surface of the star, caused 
the extraordinary increase of brilliancy. Now, the spectro- 
scope shows that the red flames of the solar chromosphere 
(197) are largely composed of hydrogen; and it is not unlikely 
that the blazing-forth of this star arose from an action similar 
to that which produces these flames, only on an immensely 
larger scale. 

Distance of the Stars. 

361. Parallax of the Stars. — Such is the distance of 
the stars, that only in a comparatively few instances has any 
displacement of these bodies been detected when viewed 
from opposite parts of the earth's orbit, that is, from points 
a hundred and eighty- five million miles apart ; and in no 
case can this displacement be detected except by the most 
careful and delicate measurement. Half of the above dis- 
placement, or the displacement of the star as seen from 
the earth instead of the sun, is called the parallax of the 
star. In no case has a parallax of one second as yet been 
detected. 

362. The Distance of the Stars. — The distance of a star 
whose parallax is one second would be 206,265 times the 
distance of the earth from the sun, or about nineteen million 
million miles. It is quite certain that no star is nearer than 
this to the earth. Light has a velocity which would carry 
it seven times and a half around the earth in a second ; but 
it would take it more than three years to reach us from 
that distance. Were all the stars blotted out of existence 
to-night, it would be at least three years before we should 
miss a single one. 

Alpha Centauri, the brightest star in the constellation 
of the Centaur, is, so far as we know, the nearest of the 
fixed stars. It is estimated that it would take its light about 
three years and a half to reach us. It has also been esti- 



ASTRONOMY, 



3^5 



mated that it would take light over sixteen years to reach 
us from Sinus, about eighteen years to reach us from Viga, 
about twenty-five years from A re turns, and over forty years 
from the Pole-Star. In many instances it is believed that 
it would take the light of stars hundreds of years to make 
the journey to our earth, and in some instances even thou- 
sands of years. 

Proper Motion of the Stars. 

363. Why the Stars appear Fixed. — The stars seem to 
retain their relative positions in the heavens from year to 




Fig. 420. 

year, and from age to age ; and hence they have come 
universally to be denominated as fixed. It is, however, now 
well known that the stars, instead of being really stationary, 
are moving at the rate of many miles a second ; but their 
distance is so enormous, that, in the majority of cases, it 
would be thousands of years before this rate of motion 
would produce a sufficient displacement to be noticeable 
to the unaided eye. 



'366 ASTRONOMY. 

364. Secular Displacement of the Stars. — Though 
proper motion of the stars is apparently slight, it will, 
the course of many age% produce a marked change in the 
configuration of the stars. Thus, in Fig. 420, the left-hand 
portion shows the present configuration of the stars of 
the Great Dipper. The small arrows attached to the stars 
show the direction and comparative magnitudes of their 
motion. The right-hand portion of the figure shows these 




Fig. 421. 

stars as they will appear thirty-six thousand years from the 
present time. 

Fig. 421 shows in a similar way the present configura- 
tion and proper motion of the stars of Cassiopeia, and 
also these stars as they will appear thirty-six thousand years 
hence. 

Fig. 422 shows the same for the constellation Orion. 

365. The Secular Motion of the Sun. — The stars in all 
parts of the heavens are found to move in all directions 
and with all sorts of velocities. When, however, the motions 



ASTRONOMY. 



& 



of the stars are averaged, there is found to be an apparent 
proper motion common to all the stars. The stars in the 




neighborhood of Hercules appear to be approaching us, 
and those in the 
opposite part of 
the heavens ap- 
pear to be re- 
ceding from us. 
In other words, 
all the stars 
appear to be 
moving away 
from Hercules, 
and towards the 
opposite part of 
the heavens. Fi s- 423. 

This apparent motion common to all the stars is held by 
astronomers to be due to the real motion of the sun 




368 



ASTRONOMY. 







through space. The point in the heavens towards which 
our sun is moving at the present time is indicated by the 
small circle in the constellation Hercules in Fig. 423. As 
the sun moves, he carries the earth and all the planets along 
with him. Fig. 424 shows the direction of the sun's motion 

with reference to the eclip- 
tic and to the axis of the 
earth. Fig. 425 shows the 
earth's path in space ; and 
Fig. 426 shows the paths of 
the earth, the moon, Mer- 
cury, Venus, and Mars in 
space. 

Whether the sun is actu- 
Fig. 424. ally moving in a straight 

line, or around some distant centre, it is impossible to deter- 
mine at the present time. It is estimated that the sun is 
moving along his path "at the rate of about a hundred and 
fifty million miles a year. This is about five-sixths of the 
diameter of the earth's 
orbit. 

366. Star -Drift. — In 
several instances, groups 
of stars have a common 
proper motion entirely dif- 
ferent from that of the 
stars around and among 
them. Such groups proba- 
bly form connected sys- 
tems, in the motion of Fl §- 42 5- 
which all the stars are carried along together without any 
great change in their relative positions. The most re- 
markable case of this kind occurs in the constellation 
Taurus. A large majority of the brighter stars in the 
region between Aldebaran and the Pleiades have a common 




ASTRONOMY. 



3^9 



proper motion of about ten seconds per century towards 
the east. Proctor has shown that five out of the seven 
stars which form the Great Dipper have a common proper 




motion, as shown in Fig. 427 (see also Fig. 420). He pro- 
poses for this phenomenon the name of Star-Drift. 




( ^ 



/ 



/ 



/ 



y*V* \ 



367. Motion of Stars along the Line of Sight. — A motion 
of a star in the direction of the line of sight would produce 
no displacement of the star that could be detected with the 



370 ASTRONOMY, 

telescope ; but it would cause a change in the brightness of 
the star, which would become gradually fainter if moving from 
us, and brighter if approaching us. Motion along the line 
of sight has, however, been detected by the use of the tele- 
spectroscope (152), owing to the fact that it causes a displace- 
ment of the spectral lines. As has already been explained 
(469), a displacement of a spectral line towards the red end 
of the spectrum indicates a motion away from us, and a dis- 
placement towards the violet end, a motion towards us. 

By means of these displacements of the spectral lines, 
Huggins has detected motion in the case of a large number 
of stars, and calculated its rate : — 

STARS RECEDING FROM US. 

Sirius 20 miles per second. 

Betelgeuse 22 miles per second. 

Rigel 15 miles per second. 

Castor 25 miles per second. 

Regulus 15 miles per second. 

STARS APPROACHING US. 

Arcturus 55 miles per second. 

Vega 50 miles per second. 

Deneb 39 miles per second. 

Pollux 49 miles per second. 

Alpha Ursae Majoris ... 46 miles per second. 

These results are confirmed by the fact that the amount 
of motion indicated is about what we should expect the 
stars to have, from their observed proper motions, combined^ 
with their probable distances. Again : the stars in the 
neighborhood of Hercules are mostly found to be approach- 
ing the earth, and those which lie in the opposite direction 
to be receding from it ; which is exactly the effect which 
would result from the sun's motion through space. The 
five stars in the Dipper, which have a common proper 



ASTRONOMY. 37 1 

motion, are also found to have a common motion in the 
line of sight. But the displacement of the spectral lines 
is so slight, and its measurement so difficult, that the veloci- 
ties in the above table are to be accepted as only an 
approximation to the true values. 

Chemical and Physical Constitution of the Stars. 

368. The Constitution of the Stars Similar to that of 
the Sun. — The stellar spectra bear a general resemblance 
to that of the sun, with characteristic differences. These 
spectra all show Fraunhofer's lines, which indicate that 
their luminous surfaces are surrounded by atmospheres con- 
taining absorbent vapors, as in the case of the sun. The 
positions of these lines indicate that the stellar atmospheres 
contain elements which are also found in the sun's, and 
on the earth. 

369. Four Types of Stellar Spectra. — The spectra of 
the stars have been carefully observed by Secchi and Hug- 
gins. They have found that stellar spectra may be reduced 
to four types, which are shown in Fig. 428. In the spec- 
trum of Sinus, a representative of Type I, very few lines 
are represented ; but the lines are very thick. 

Next we have the solar spectrum, which is a repre- 
sentative of Type II, one in which more lines are rep- 
resented. In Type III. fluted spaces begin to appear, 
and in Type IV., which is that of the red stars, nothing 
but fluted spaces is visible ; and this spectrum shows that 
something is at work in the atmosphere of those red 
stars different from what there is in the simpler atmosphere 
of Type I. 

Lockyer holds that these differences of spectra are due 
simply to differences of temperature. According to him, 
the red stars, which give the fluted spectra, are of the 
lowest temperature ; and the temperature of the stars of 



372 ASTRONOMY. 

the different types gradually rises till we reach the first 
type, in which the temperature is so high that the dis- 



Fig. 428. 

sociation (161) of the elements is nearly if not quite 
complete. 



ASTRONOMY. 



373 



III. NEBULA. 

Classification of Nebulae. 

370. Planetary Nebula. — Many nebulae (328) present a 
well-defined circular disk, like that of a planet, and are there- 
fore called planetary nebulae. Specimens of planetary neb- 
ulae are shown in Fig. 429. 

371. Circular and Elliptical Nebula. — While many 
nebulae are circular in form, others are elliptical. The for- 
mer are called circular nebulae, and the latter elliptical 
nebulae. Elliptical nebulae have been discovered of every 
degree of eccentricity. Examples of various circular and 
elliptical nebulae are given in Fig. 430. 



1 




B 



Fig. 429. 

372. Annular Nebula. — Occasionally ring-shaped nebu- 
lae have been observed, sometimes with, and sometimes 
without, nebulous matter within the ring. They are called 
annular nebulae. They are both circular and elliptical in 
form. Several specimens of this class of nebulae are given 
in Fig. 431. 

373. Nebulous Stais. — Sometimes one or more minute 
stars are enveloped in a nebulous haze, and are hence 
called nebulous stars. Several of these nebulae are shown 
in Fig. 432. 

374. Spiral Nebula. — Very many nebulae disclose a 
more or less spiral structure, and are known as spiral 
nebulae. They are illustrated in Fig. 433. There are, how- 



374 



ASTRONOMY. 



: 1 





3 
G 

9 

/'■■' 


4 


5 


7 


8 



Fig. 430. 




ASTRONOMY. 



375 



ever, a great variety of spiral forms. We shall have occa- 
sion to speak of these nebulae again (381-383). 

375. Double and Multiple Neb u Ice. — Many double and 



1 2 




4 5 

5 


6 



Fig. 432. 

multiple nebulae have been observed, some of which are 
represented in Fig. 434. 

Fig. 435 shows what appears to be a double annular 
nebula. Fig. 436 gives two views of a double nebula. 



#®F 


1& 


f 

1 



Fig. 433- 



The change of position in the components of this double 
nebula indicates a motion of revolution similar to that of 
the components of double stars. 



376 



ASTRONOMY. 



Irregular Nebulae. 

376. Irregular Forms. — Besides the more or less regu- 
lar forms of nebulae which have been classified as indicated 



1 


2 


* 


* 






4 


3 





Fig. 437 shows 



Fig. 434- 

above, there are many of very irregular shapes, and some 
of these are the most remarkable nebulae in the heavens. 
a curiously shaped nebula, seen by Sir 
John Herschel in the southern 
heavens ; and Fig. 438, one in 
Taurus\ known as the Crab 
nebula. 

377. The Great Nebula of 

Andromeda. — This is one of 

the few nebulae that are visible 

to the naked eye. We see at 

Fig. 435. a glance that it is not a star, 

but a mass of diffused light. Indeed, it has sometimes 

been very naturally mistaken for a comet. It was first 

described by Marius in 16 14, who compared its light to 







ASTRONOMY. 



377 



that of a candle shining through horn. This gives a very 
good idea of the impression it produces, which is that of 
a translucent object illuminated by a brilliant light behind 
it. With a small 
telescope it is easy 
to imagine it to be 
a solid like horn ; 
but with a large one 
the effect is more 
like fog or mist with 
a bright body in its 
midst. Unlike most 
of the nebulae, its 
spectrum is a con- 
tinuous one, similar Fig. 43 6. 
to that from a heated solid, indicating that the light 
emanates, not from a glowing gas, but from matter in the 
solid or liquid state. This would suggest that it is really 





Fig. 437- 

an immense star-cluster, so distant that the highest tele- 
scopic power cannot resolve it ; yet in the largest telescopes 
it looks less resolvable, and more like a gas, than in 
those of moderate size. If it is really a gas, and if the 



378 ASTRONOMY. 

spectrum is continuous throughout the whole extent of 
the nebula, either it must shine by reflected light, or the 
gas must be subjected to a great pressure almost to its 
outer limit, which is hardly possible. If the light is re- 
flected, we cannot determine whether it comes from a single 




bright star, or a number of small ones scattered through 
the nebula. 

With a small telescope this nebula appears elliptical, as 
in Fig. 439. Fig. 440 shows it as it appeared to Bond, in 
the Cambridge refractor. 

378. The Great Nebula of Orion. — The nebula which, 
above all others, has occupied the attention of astrono- 



ASTRONOMY. 



379 







£c* 






55* ^ ■ --'jw 


^f&Tl WLlY^W't 1 'HfP^ 


W^ : '- 


;'<JjjH 




»£-■' b5 





Fig. 439- 




Fig. 440. 



3 8o 



ASTRONOMY, 




Fig. 441. 



ASTRONOMY. 



381 



mers, and excited the wonder of observers, is the %reat 
nebula of Orion, which surrounds the middle star of the 
three which form the sword of Orion. A good eye will per- 
ceive that this star, instead of looking like a bright point, 
has a hazy appearance, due to the surrounding nebula. 
This object was first described by Huyghens in 1659. as 
follows : — 

" There is one phenomenon among the fixed stars worthy 
of mention, which, so far as I know, lias hitherto been 




Fig. 442. 

noticed by no one, and indeed cannot be well observed 
except with large telescopes. In the sword of Orion are 
three stars quite close together. In 1656, as I chanced 
to be viewing the middle one of these with the telescope, 
instead of a single star, twelve showed themselves (a not 
uncommon circumstance). Three of these almost touched 
each other, and with four others shone through a nebula, 
so that the space around them seemed far brighter than the 
rest of the heavens, which was entirely clear, and appeared 



382 



ASTRONOMY. 



quite black ; the effect being that of an opening in the sky, 

through which a brighter region was visible." 

The representation 
of this nebula in Fig. 
441 is from a drawing 
made by Bond. In 
brilliancy and variety 
of detail it exceeds 
any other nebula visi- 
ble in the northern 
hemisphere. In its 
centre are four stars, 
easily distinguished by 
Fig. 443. a small telescope with 

a magnifying power of forty or fifty, together with two 





Fig. 444. 

smaller ones, requiring a nine-inch telescope to be well seen. 
Besides these, the whole nebula is dotted with stars. 



ASTRONOMY. 



383 



In the winter of 1864-65 the spectrum of this nebula 
was examined independently by Secchi and Huggins, who 
found that it consisted of three bright lines, and hence 
concluded that the nebula was composed, not of stars, but 
of glowing gas. The position of one of the lines was near 
that of a line of nitrogen, while another seemed to coin- 
cide with a hydrogen line. This would suggest that the 
nebula is a mixture of hydrogen and nitrogen gas ; but of 
this we cannot be certain. 

379. The Nebula in Argus. — There is a nebula (Fig. 
442) surrounding 
the variable star 
Eta Argus (355), 
which is remarka- 
ble as exhibiting 
variations of bright- 
ness and of out- 
line. 

In many other 
nebulae, changes 
have been suspect- 
ed ; but the indis- 
tinctness of outline 
which characterizes 
most of these ob- 
jects, and the very different aspect they present in telescopes 
of different powers, render it difficult to prove a change 
beyond a doubt. 

380. The Dumb- Bell Nebula, — This nebula was named 
from its peculiar shape. It is a good illustration of the 
change in the appearance of a nebula when viewed with 
different magnifying powers. Fig. 443 shows it as it ap- 
peared in Herschel's telescope, and Fig. 444 as it appears 
in the great Parsonstown reflector (20). 




3^4 



ASTRONOMY. 



Spiral Nebula. 

381. The Spiral Nebula in Canes Venatici. — The grea 
spiral nebula in the constellation Canes Venatici, or the 




Fig. 446. 

Hunting-Dogs, is one of the most remarkable of its class. 
Fig. 445 shows this nebula as it appeared in HerschePs 
telescope, and Fig. 446 shows it as it appears in the Par- 
sonstown reflector. 



ASTRONOMY. 385 

382. Condensation of Nebula. — The appearance of the 




Fig. 447- 

nebula just mentioned suggests a body rotating on its axis, 
and undergoing condensation at the same time. 




Fig. 448. 



It is now a generally received theory that nebulae are the 
material out of which stars are formed. According to this 



386 



ASTRONOMY. 



theory, tne stars originally existed as nebulae, and all nebulse 
will ultimately become condensed into stars. 




Fig. 449. 



383. Other Spiral Nebula. — Fig. 447 represents a spiral 



ASTRONOMY. 



387 



nebula of the Great Bear. This nebula seems to have sev- 
eral centres of condensation. Fig. 448 is a view of a spiral 











': 1 


'- -i*^$^*J|- 




%^ 








j 



Fig. 450. 

nebula in Cepheus, and Fig. 449 of a singular spiral nebula 
in the Triangle. This also appears to have several points 




Fig. 45 



of condensation. Figs. 450 and 451 represent oval and 
elliptical nebulae having a spiral structure. 



388 



ASTRONOMY. 




Fig. 45*- 



ASTRONOMY. 



389 



THE MAGELLANIC CLOUDS. 

384. Situation and General Appearance of the Magel- 
lanic Clouds. — The Magellanic clouds are two nebulous- 
looking bodies near the 
southern pole of the heav- 
ens, as shown in the right- 
hand portion of Fig. 452. 
In the appearance and 
brightness of their light 
they resemble portions of 
the Milky-Way. 

The larger of these 
clouds is called the Nu- 
becula Major. It is visi- 
ble to the naked eye in 
strong moonlight, and cov- 
ers a space about two 
hundred times the surface 




oi 



Fig. 453- 

the moon. It is shown in 
Fig. 453. The smaller 
cloud is called the Nu- 
becula Minor. It has 
only about a fourth the 
extent of the larger cloud, 
and is considerably less 
brilliant. It is visible to 
the naked eye, but it dis- 
appears in full moonlight. 
This cloud is shown in 
Fig. 454. The region 
around this cloud is sin- 
gularly bare of stars ; but 
the magnificent cluster of 

Toucan, already described (346), is near, and is shown a 

little to the right of the cloud in the figure. 




390 ASTRONOMY. 

385. Structure of the Nubecula. — Fig. 455 shows the 
structure of these clouds as revealed by a powerful tele- 
scope. The general ground of both consists of large tracts 
and patches of nebulosity in every stage of resolution, — 
from that which is irresolvable with eighteen inches of 
reflecting aperture, up to perfectly separated stars, like the 
Milky- Way and clustering groups. There are also nebulae 
in abundance, both regular and irregular, globular clusters 




Fig. 455- 

in every state of condensation, and objects of a nebulous 
character quite peculiar, and unlike any thing in other 
regions of the heavens. In the area occupied by the 
nubecula major two hundred and seventy-eight nebulae and 
clusters have been enumerated, besides fifty or sixty outliers, 
which ought certainly to be reckoned as its appendages, 
being about six and a half per square degree ; which very 
far exceeds the average of any other part of the nebulous 
heavens. In the nubecula nii?ior the concentration of such 
objects is less, though still very striking. The nubeculae, 



ASTRONOMY. ^q F 

then, combine, each within its own area, characters which 
in the rest of the heavens are no less strikingly separated ■ 
namely, those of the galactic and the nebular system. 
Globular clusters (except in one region of small extent) and 
nebulae of regular elliptic forms are comparatively rare in 
the Milky-Way, and are found congregated in the greatest 
abundance in a part of the heavens the most remote possi- 
ble from that circle; whereas in the nubeculae they are 
indiscriminately mixed with the general starry ground, and 
with irregular though small nebulae. 

The Nebular Hypothesis. 

386. The Basis of the A T ebular Hypothesis. — We have seen 
that the planets all revolve around the sun from west to east 
in nearly the same plane, and that the sun rotates on his axis 
from west to east. The planets, so far as known, rotate on 
their axes from west to east; and all the moons, except those 
of Uranus and Neptune, revolve around their planets from 
west to east. These common features in the motion of the 
sun, moons, and planets, point to the conclusion that they are 
of a common origin. 

387. Kanfs Hypothesis. — Kant, the celebrated German 
philosopher, seems to have the best right to be regarded as the 
founder of the modern nebular hypothesis. His reasoning has 
been concisely stated thus : " Examining the solar system, we 
find two remarkable features presented to our consideration. 
One is, that six planets and nine satellites [the entire number 
then known] move around the sun in circles, not only in the 
same direction in which the sun himself revolves on his axis, 
but very nearly in the same plane. This common feature of 
the motion of so many bodies could not by any reasonable 
possibility have been a result of chance : we are therefore 
forced to believe that it must be the result of some common 
cause originally acting on all the planets. 

" On the other hand, when we consider the spaces in which 
the planets move, we find them entirely void, or as good as 
void; for, if there is any matter in them, it is so rare as to be 



392 



ASTRONOMY. 



without effect on the planetary motions. There is, therefore, 
no material connection now existing between the planets 
through which they might have been forced to take up a 
common direction of motion. How, then, are we to reconcile 
this common motion with the absence of all material connec- 
tion ? The most natural way is to suppose that there was once 
some such connection, which brought about the uniformity of 
motion which we observe ; that the materials of which the 
planets are formed once filled the whole space between them. 
There was no formation in this chaos, the formation of sepa- 
rate bodies by the mutual gravitation of parts of the mass 
being a later occurrence. But, naturally, some parts of the 
mass would be more dense than others, and would thus gather 
around them the rare matter which filled the intervening spaces. 
The larger collections thus formed would draw the smaller 
ones into them, and this process would continue until a few 
round bodies had taken the place of the original chaotic 
mass." 

Kant, however, failed to account satisfactorily for the motion 
of the sun and planets. According to his system, all the 
bodies formed out of the original nebulous mass should have 
been drawn to a common centre so as to form one sun, instead 
of a system of revolving bodies like the solar system. 

388. HerscheVs Hypothesis. — The idea of the gradual trans- 
mutation of nebulae into stars seems to have been suggested 
to Herschel, not by the study of the solar system, but by that 
of the nebulae themselves. Many of these bodies he believed 
to be immense masses of phosphorescent vapor ; and he con- 
ceived that these must be gradually condensing, each around 
its own centre, or around the parts where it is most dense, 
until it should become a star, or a cluster of stars. On classi- 
fying the nebulae, it seemed to him that he could see this 
process going on before his eyes. There were the large, faint, 
diffused nebulae, in which the condensation had hardly begun ; 
the smaller but brighter ones, which had become so far con- 
densed that the central parts would soon begin to form into 
stars ; yet others, in which stars had actually begun to form ; 
and, finally, star-clusters in which the condensation was com- 
plete. The spectroscopic revelations of the gaseous nature of 






ASTRONOMY. 



393 



the true nebulae tend to confirm the theory of Herschel, that 
these masses will all, at some time, condense into stars. 

389. Laplace's Hypothesis. — Laplace was led to the nebular 
hypothesis by considering the remarkable uniformity in the 
direction of the rotation of the planets. Believing that this 
could not have been the result of chance, he sought to investi- 
gate its cause. This, he thought, could be nothing else than 
the atmosphere of the sun, which once extended so far out as 
to fill all the space now occupied by the planets. He begins 
with the sun, surrounded by this immense fierv atmosphere. 
Since the sum total of rotary motion now seen in the planetary 
system must have been there from the beginning, he conceives 
the immense vaporous mass forming the sun and his atmos- 
phere to have had a slow rotation on its axis. As the intensely 
hot mass gradually cooled, it would contract towards the centre. 
As it contracted, its velocity of rotation would, by the laws of 
mechanics, constantly increase: so that a time would arrive, 
when, at the outer boundary of the mass, the centrifugal force 
due to the rotation would counterbalance the attractive force 
of the central mass. Then those outer portions would be left 
behind as a revolving ring, while the next inner portions would 
continue to contract until the centrifugal and attractive forces 
were again balanced, when a second ring would be left behind : 
and so on. Thus, instead of a continuous atmosphere, the sun 
would be surrounded by a series of concentric revolving rings 
of vapor. As these rings cooled, their denser materials would 
condense first ; and thus the ring would be composed of a 
mixed mass, partly solid and partly vaporous, the quantity of 
solid matter constantly increasing, and that of vapor diminish- 
ing. If the ring were perfectly uniform, this condensation 
would take place equally all around it, and the ring would thus 
be broken up into a group of small planets, like the asteroids. 
But if, as would more likely be the case, some portions of the 
ring were much denser than others, the denser portions would 
gradually attract the rarer portions, until, instead of a ring, 
there would be a single mass composed of a nearly solid 
centre, surrounded by an immense atmosphere of fiery vapor. 
This condensation of the ring of vapor around a single point 
would not change the amount of rotary motion that had existed 



394 ASTRONOMY. 

in the ring. The planet with its atmosphere would there- 
fore be in rotation ; and would be, on a smaller scale, like the 
original solar mass surrounded by its atmosphere. In the 
same way that the latter formed itself first into rings, which 
afterwards condensed into planets, so the planetary atmos- 
pheres, if sufficiently extensive, would form themselves into 
rings, which would condense into satellites. In the case of 
Saturn, however, one of the rings was so uniform throughout, 
that there was no denser portion to attract the rest around it; 
and thus the ring of Saturn retained its annular form. 




Fig. 456. 

Such is the celebrated nebular hypothesis of Laplace. It 
starts, not with a purely nebulous mass, but with the sun, sur- 
rounded by an immense atmosphere, out of which the planets 
were formed by gradual condensation. Fig. 456 represents 
the condensing mass according to this theory. 

390. The Modern Nebular Hypothesis. — According to the 
nebular hypothesis as held at the present time, the sun, plan- 
ets, and meteoroids originated from a purely nebulous mass. 
This nebula first condensed into a nebulous star, the star being 
the sun, and its surrounding nebulosity being the fiery atmos- 
phere of Laplace. The original nebula must have been put 
into rotation at the beginning. As it contracted and became 



ASTRONOMY. 395 

condensed through the loss of heat by radiation into space, 
and under the combined attraction of gravity, cohesion, and 
affinity, its speed of rotation increased ; and the nebulous 
envelop became, by the centrifugal force, flattened into a thin 
disk, which finally broke up into rings, out of which were 
formed the planets and their moons. According to Laplace, 
the rings which were condensed into the planets were thrown 
off in succession from the equatorial region of the condensing 
nebula; and so the outer planets would be the older. Accord- 
ing to the more modern idea, the nebulous mass was first flat- 
tened into a disk, and subsequently broken up into rings, in 
such a way that there would be no marked difference in the 
ages of the planets. The sun represents the central portion 
of the original nebula, and the comets and meteoroids its out- 
lying portion. At the sun the condensation is still going on, 
and the meteoroids appear to be still gradually drawn in to 
the sun and planets. 

The whole store of energy with which the original solar 
nebula was endowed existed in it in the potential form. By 
the condensation and contraction this energy was gradually 
transformed into the kinetic energy of molar motion and of 
heat; and the heat became gradually dissipated by radiation 
into space. This transformation of potential energy into heat 
is still going on at the sun, the centre of the condensing mass, 
by the condensation of the sun itself, and by the impact of 
meteors as they fall into it. 

It has been calculated, that, by the shrinking of the sun to 
the density of the earth, the transformation of potential energy 
into heat would generate enough heat to maintain the sun's 
supply, at the present rate of dissipation, for seventeen million 
years. A shrinkage of the sun which would generate all the 
heat he has poured into space since the invention of the tele- 
scope could not be detected by the most powerful instruments 
yet constructed. 

The least velocity with which a meteoroid could strike the 
sun would be two hundred and eighty miles a second ; and 
it is easy to calculate how much heat would be generated 
by the collision. It has been shown, that, were enough meteo- 
roids to fall into the sun to develop its heat, they would not 



396 ASTRONOMY. 

increase his mass appreciably during a period of two thousand 
years. 

The sun's heat is undoubtedly developed by contraction and 
the fall of meteoroids ; that is to say, by the transformation of 
the potential energy of the original nebula into heat. 

It must be borne in mind that the nebular hypothesis is sim- 
ply a supposition as to the way in which the present solar 
system may have been developed from a nebula endowed with 
a motion of rotation and with certain tendencies to condensa- 
tion. Of course nothing could have been developed out of 
the nebula, the germs of which had not been originally im- 
planted in it by the Creator. 

IV. THE STRUCTURE OF THE STELLAR 
UNIVERSE. 

391. Sir William HerscheVs View. — Sir William Herschel 
assumed that the stars are distributed with tolerable uniformity 
throughout the space occupied by our stellar system. He 




Fig. 457- 
accounted for the increase in the number of stars in the field 
of view as he approached the plane of the Milky-Way, not 
by the supposition that the stars are really closer together in 
and about this plane, but by the supposition that our stellar 
system is in the form of a flat disk cloven at one side, and 
with our sun near its centre. A section of this disk is shown 
in Fig. 457. 



ASTRONOMY. 



397 



An observer near S, with his telescope pointed in the direc- 
tion of Sfr, would see comparatively few stars within the field 
of view, because looking through a comparatively thin stratum 
of stars. With his telescope pointed in the direction Sa, he 
would see many more stars within his field of view, even though 
the stars were really no nearer together, because he would be 
looking through a thicker stratum of stars. As he directed 
his telescope more and more nearly in the direction Sf, he 
would be looking through a thicker and thicker stratum of 
stars, and hence he would see a greater and greater number of 
them in the field of view, though they were everywhere in the 
disk distributed at uniform distances. He assumed, also, that 
the stars are all tolerably 
uniform in size, and that 
certain stars appear small- 
er than others, only be- 
cause they are farther off. 
He supposed the faint 
stars of the Milky- Way 
to be merely the most dis- 
tant stars of the stellar 
disk ; that they are really 
as large as the other 
stars, but appear small 
owing to their great dis- 
tance. The disk was as- 
sumed to be cloven on Fi s- 458. 
one side, to account for the division of the Milky-Way through 
nearly half of its course. This theory of the structure of the 
stellar universe is often referred to as the cloven disk theory. 

392. The Cloven Ring Theory. — According to Madler, the 
stars of the Milky- Way are entirely separated from the other 
stars of our system, belonging to an outlying ring, or system 
of rings. To account for the division of the Milky- Way, the 
ring is supposed to be cloven on one side : hence this theory 
is often referred to as the cloven ring theory. According to 
this hypothesis, the stellar system viewed from without would 
present an appearance somewhat like that in Fig. 458. 1 he 
outlying ring cloven on one side would represent the stars 




398 






ASTRONOMY. 



of the Milky-Way ; and the luminous mass at the centre, the 
remaining stars of the system. 

393. Proctor's View. — According to Proctor, the Milky- 
Way is composed of an irregular spiral stream of minute stars 
lying in and among the larger stars of our system, as repre- 
sented in Fig. 459. The spiral stream is shown in the inner 




Fig. 459- 

circle as it really exists among the stars, and in the outer 
circle as it is seen projected upon the sky. According to this 
view, the stars of the Milky- Way appear faint, not because 
they are distant, but because they are really small. 

394. Newcomfrs View. — According to Newcomb, the stars 
of our system are all situated in a comparatively thin zone 
lying in the plane of the Milky-Way, while there is a zone 
of nebulae lying on each side of the stellar zone. He believes 



ASTRONOMY. 



399 



that so much is certain with reference to the structure of our 
stellar universe ; but he considers that we are as yet compara- 




Fig. 460. 

tively ignorant of the internal structure of either the stellar 
or the nebular zones. The structure of the stellar universe, 
according to this view, is shown in Fig* 460. 



INDEX. 



A. 

Aberration of light, 38. 

Aerolites, 304. 

Aldebaran, star in Taurus, 340, 342. 

Algol, a variable star, 343, 358. 

Almanac, perpetual, 82. 

Alps, lunar mountains, 126. 

Altair, star in Aquila, 336. 

Alt-azimuth instrument, 13. 

Altitude, 12. 

Andromeda (constellation), 343, 346. 

nebula in, 376. 
Angstrom's map of spectrum, 164. 
Antares, star in Scorpio, 347. 
Apennines, lunar mountains, 122, 124. 
Apheiion, 47. 
Apogee, 44. 
Aquarius, or the Water-Bearer, 350. 

cluster in, 354. 
Aquila, or the Eagle, 336. 
Arcturus, star in Bootes, 335, 365, 370. 
Argo, or the Ship, 360. 

nebula in, 383. 

variable star in, 360. 
Aries, or the Ram, 350. 
Asteroids, 223, 241. 
Astrsea, an asteroid, 241. 
Auriga, or the Wagoner, 342. 
Azimuth, 13. 



B. 

Betelgeuse, star in Orion, 340, 370. 
Berenice's Hair (constellation), 334. 
Bode's law, 241. 

disproved, 273. 
Bootes (constellation), 334, 335, 



Calendar, the, 80. 

Callisto, moon of Jupiter, 250. 

Cancer, or the Crab, 350. 

tropic of, 61. 
Canes Venatici, or the Hunting-Dogs, 



Canes Venatici, nebula in, 384. 
Canis Major, or the Great Dog, 342. 
Canis Minor, or the Little Dog, 340. 
Capella, star in Auriga, 340, 343. 
Capricorn, tropic of, 61. 
Capricornus, or the Goat, 350. 
Cassiopeia (constellation), 332. 

new star in, 362. 
Castor, star in Gemini, 340, 370. 
Caucasus, a lunar range, 124. 
Centaurus, star-cluster in, 355. 
Cepheus (constellation), 334. 

nebula in, 387. 
Ceres, the planet, 241. 
Cetus, or the Whale, 346. 

variable star in, 359. 
Charles's Wain, 330. 
Circles, great, 4. 
diurnal, 8. 
hour, 16. 
small, 4. 
vertical, 12. 
Clock, astronomical, 18. 

time, 78. 
Coma Berenices, or Berenice's Hair, 334 
Comet, Biela's, 293. 

and earth, collision of, 316, 
Coggia's, 297. 
Donati's, 296. 
Encke's, 293. 
Halley's, 291. 
of 1680, 290. 
of 1811, 290, 
of 1843, 295- 
of 1861, 297. 
of June, 1881, 300. 
Comets, appearance of, 274. 
and meteors, 313. 
bright, 274. 

chemical constitution of, 318. 
development of, 277. 
number of, 288. 
orbits of, 282. 
origin of, 287. 
periodic, 286. 

physical constitution of, 314. 
tails of, 279. 
telescopic, 275, 281. 
visibility of, 281. 
Conic sections, 48. 

401 



402 



INDEX. 



Conjunction, 91. 

inferior, 130. 
superior, 130, 136. 
Constellations, 325. 

zodiacal, 32. 
Copernican system, the, 44, 53. 
Copernicus, a lunar crater, 120, 129. 
Corona Borealis, or the Northern Crown, 

336. 
Corona Borealis, new star in, 363. 
Corvus, or the Crow, 339. 
Crystalline spheres, 41. 
Cycles and epicycles, 42. 
Cygnus, or the Swan, 338. 

D. 

Day and night, 57. 

civil, 77. 

lunar, 108. 

sidereal, 74. 

solar, 74. 
Declination, 16. 
Deimos, satellite of Mars, 239. 
Delphinus, or the Dolphin, 338. 
Deneb, star in Cygnus, 338, 370. 
Dione, satellite of Saturn, 259. 
Dipper, the Great, 330, 366, 369, 370. 
the Little, 331. 
the Milk, 347. 
Dissociation, 163. 
Dominical Letter, the, 81. 
Draco, or the Dragon, 331. 



E. 

Earth, density of, 85. 

flattened at poles, 55. 
form of, 53. 
in space, 56. 
seen from moon, 109. 
size of, 55. 
weight of, 83. 
Eccentric, the 43. 
Eccentricity, 46. 
Eclipses, 210. 

annular, 219. 
lunar, 210, 214. 
solar, 216. 
Ecliptic, the, 27. 

obliquity of, 28. 
Ellipse, the 45, 49. 
Elongation, of planet, 130. 
Enceladus, moon of Saturn, 259. 
Epicycles, 42. 
Epicycloid, 107. 

Epsilon Lyrse, a double star, 356. 
Equator, the celestial, 7. 
Equinoctial, the, 7. 

colure, 16. 
elevation of, 9. 
Equinox, autumnal, 29. 
vernal, 16, 29. 
Equinoxes, precession of, 31, 85. 
Eta Argus, a variable star, 360, 383. 
Europa, moon of Jupiter, 250. 



F. 

Faculae, solar, 177. 

Fomalhaut, star in Southern Fish, 350. 

Fraunhofer's lines, 164, 371. 



Galaxy, the, 326. 
Ganymede, moon of Jupiter, 250. 
Gemini, or the Twins, 340. 
Georgium Sidus, 271. 



H. 

Hercules (constellation), 336. 

cluster in, 353. 

orbit of double star in, 357. 

solar system moving towards, 
367. 
Herschel, the planet (see Uranus). 
Herschel's hypothesis, 392, 396. 
Horizon, the, 5. 
Hyades, the, 342, 350. 
Hydra, or the Water-Snake, 340. 
Hyperbola, the, 49. 
Hyperion, moon of Saturn, 259. 



Io, moon of Jupiter, 250. 
Irradiation, 90, 113. 



J- 

Japetus, moon of Saturn, 259. 
Job's Coffin (asterism), 338. 
Juno, the planet, 241. 
Jupiter, apparent size of, 245. 

distance of, 245. 

great red spot of, 249. 

orbit of, 244. 

periods of, 246. 

physical constitution of, 246. 

rotation of, 248. 

satellites of, 250. 

eclipses of, 252. 
transits of, 254. 

volume of, 245. 

without satellites, 255. 



K. 

Kant's hypothesis, 391. 
Kepler, a lunar crater, 129. 
Kepler's system, 44. 

laws, 46. 

star, 362. 
Kirchhoff' s map of spectrum, 164. 



INDEX. 



403 



Laplace's hypothesis, 392. 
Latitude, celestial, 30. 
Leap year, 81. 
Leo, or the Lion, 334. 
Leonids (meteors), 312. 
Libra, or the Balances, 347. 
Libra tion, 102. 
Longitude, celestial, 30. 
Lyra, or the Lyre, 338. 
double star in, 356. 



M. 



Magellanic clouds, the, 389. 
Magnetic storms, 190. 
Magnetism and sun-spots, 190. 
Mars, apparent size of, 236. 
brilliancy of, 237. 
distance of, 235. 
orbit of, 235. 
periods of, 237. 
rotation of, 239. 
satellites of, 239. 
volume of, 236. 
Mercury, apparent size of, 226. 
atmosphere of, 228. 
distance of, 225. 
elongation of, 227. 
orbit of, 225. 
periods of, 227. 
volume of, 226. 
Meridian, the, 12. 
Meridian circle, 17. 
Meridians, celestial, 31. 
Meteoric iron, 305, 307. 
showers, 310. 
stones, 305. 
Meteors, 300. 

August, 311. 
light of, 309. 
November, 312. 
sporadic, 310. 
Meteoroids, 308. 
Micrometers, 20, 153. 
Milky- Way, the, 326. 
Mimas, moon of Saturn, 259. 
Mira, a variable star, 359. 
Moon, apparent size of, 87, 89. 
aspects of, 91. 
atmosphere of, 109. 
chasms in, 123. 
craters in, 119. 
day of, 108. 
distance of, 86. 
eclipses of, 210. 
form of orbit, 97. 
harvest, 101. 
hunter's, 102. 
inclination of orbit, 97. 
kept in her path by gravity, 51. 
librations of, 102. 
mass of, 90. 

meridian altitude of, 98. 
mountains of, 116. 



Moon, orbital motion of, 91. 
phases of, 93. 
real size of, 88. 
rising of, 99. 
rotation of, 102. 
sidereal period of, 92. 
surface of, 115. 
synodical period of, 92. 
terminator of, 115. 
wet and dry, 98. 



N. 



Nadir, the, 6. 

Neap-tides, 72. 

Nebula, in Andromeda, 376. 

crab, 376. 

dumb-bell, 383. 

in Argus", 383. 

in Canes Venatici, 384. 

in Cepheus, 387. 

in Orion, 378. 

in the Triangle, 387. 

in Ursa Major, 386. 
Nebulae, 281, 330, 373. 

annular, 373. 

circular, 373. 

condensation of, 385. 

double, 375. 

elliptical, 373. 

irregular, 376. 

multiple, 375. 

spiral, 373, 384. 
Nebular hypothesis, the, 391. 
Neptune, discovery of, 271. 
orbit of, 271. 
satellite of, 274. 
New style, 80. 
Newcomb's theory of the stellar universe, 

398. 
Newton's system, 48. 
Nodes, 97. 
Nubecula, Major, 389. 

Minor, 389. 
Nutation, 34. 



O. 

Olbers's hypothesis, 241. 

Old style, 80. 

Ophiuchus (constellation), 347. 

new star in, 362. 
Opposition, 91, 136. 
Orion, 341. 

nebula in, 378. 

the trapezium of, 356. 



Pallas, the planet, 241. 
Parabola, the, 49. 
Parallax, 37. 

Pegasus (constellation), 343, 346. 
triple star in, 356. 



404 



INDEX. 



Perigee, 44. 
Perihelion, 47. 
Perseids (meteors), 311. 
Perseus (constellation), 346. 

cluster in, 353. 
Phobos, satellite of Mars, 239. 
Pico, a lunar mountain, 127. 
Pisces, or the Fishes, 350. 
Piscis Australis, or the Southern Fish, 

35o. 
Planets, 39. 

inferior, 130. 

periods of, 132. 
phases of, 132. 

inner group of, 221. 

intra-Mercurial, 230. 

minor, 223. 

outer group of, 222, 244. 

superior, 134. 

motion of, 134. 
periods of, 137. 
phases of, 137. 

three groups of, 221. 
Pleiades, the, 328, 342, 351. 
Pointers, the, 330. 
Polar distance, 16. 
Pole Star, the, 7, 330, 365. 
Poles, celestial, 7, 9. 
Pollux, star in Gemini, 340, 370. 
Praesepe, or the Beehive, 350. 
Precession of equinoxes, 31, 85. 
Prime vertical, the, 12. 
Proctor's theory of the stellar universe, 

398. 
Procyon, star in Canis Minor, 340. 
Ptolemaic system, the, 41. 



Q- 

Quadrature, 91, 137. 

R. 

Radiant point (meteors), 310. 
Radius vector, 47. 
Refraction, 35. 

Regulus, star in Leo, 334, 370. 
Rhea, moon of Saturn, 259. 
Rigel, star in Orion, 340, 370. 
Right ascension, 16. 

S. 

Sagittarius, or the Archer, 347. 
Saturn, apparent size of, 256. 
distance of, 256. 
orbit of, 255. 
periods of, 256. 
physical constitution of, 257. 
ring of, 261. 

changes in, 268. 
constitution of, 269. 
phases of, 263. 



Saturn, rotation of, 258. 

satellites of, 259. 

volume of, 256. 
Scorpio, or the Scorpion, 347. 

cluster in, 355. 
Seasons, the, 64. 
Sirius, the Dog-Star, 340, 342, 365, 370, 

., 37 1 - 

Solar system, the, 41. 

Solstices, 29, 59, 60. 

Sound, effect of motion on, 168. 

Spectra, bright-lined, 158. 

comparison of, 154. 
continuous, 158. 
displacement of lines in, 171, 
of comets, 318. 
reversed, 161. 
sun-spot, 193. 
types of stellar, 371. 
Spectroscope, the, 152. 

diffraction, 157. 
direct-vision, 155. 
dispersion, 152. 
Spectrum analysis, 159. 

solar, 164. 
Sphere, defined, 3. 

the celestial, 5. 

rotation of, 7. 
Spring-tides, 72. 
Stars, circumpolar, 7. 

clusters of, 328, 350. 
color of, 357. 
constellations of, 325. 
constitution of, 371. 
distance of, 364. 
double, 355. 
drift of, 368. 
four sets of, 10. 
magnitude of, 322. 
motion of, in line of sight, 369. 
multiple, 356. 
names of, 325. 
nebulous, 373. 
new, 361. 
number of, 323. 
parallax of, 364. 
proper motion of, 365. 
secular displacement of, 366. 
temporary, 361. 
variable, 358. 
Sun, atmosphere of, 149. 
brightness of, 151. 
chemical constitution of, 164. 
chromosphere of, 149, 196. 
corona of, 149, 196, 204. 
distance of, 142. 
faculse of, 177. 
heat radiated by, 150. 
inclination of axis of, 187. 
mass of, 140. 

motion of, among the stars, 26. 
at surface of, 168. 
in atmosphere of, 172. 
secular, 366. 
photosphere of, 149, 175. 
prominences of, 149, 197. 
rotation of, 186. . 



INDEX. 



405 



Sun, spectrum of, 164, 171. 
temperature of, 149. 
volume of, 140. 
winds on, 174. 
Sun-spots, 179. 

and magnetism, 190. 

birth and decay of, 185. 

cause of, 194. 

cyclonic motion in, 182. 

distribution of, 188. 

duration of, 181. 

groups of, 181. 

periodicity of, 189. 

proper motion of, 187. 

size of, 181. 

spectrum of, 193. 



T. 

Taurus, or the Bull, 342. 

quadruple star in, 356. 
Telescope, Cassegrainian, 23. 

equatorial, 19. 

front- view, 22. 

Gregorian, 23. 

Herschelian, 22. 

Lord Rosse's, 25. 

Melbourne, 25. 

Newall, 20. 

Newtonian, 22. 

Paris, 26. 

reflecting, 21. 

Washington, 20. 

Vienna, 20. 
Telespectroscope, the, 155. 
Telluric lines of spectrum, 165. 
Tethys, moon of Saturn, 259. 
Tides, 67. 
Time, clock, 78. 

sun, 78. 
Titan, moon of Saturn, 259, 261. 
Toucan, star cluster in, 354, 389. 
Transit instrument, 17. 
Transits of Venus, 145. 
Triesneker, lunar formation, 123. 
Tropics, 61. 
Twilight, 62. 
Tycho Brahe's star, 361. 

system, 44. 
Tycho, a lunar crater, 129. 



u. 

Universe, structure of the stellar, 396. 
Uranus, discovery of, 271. 

name of, 270. 

orbit of, 269. 

satellites of, 271. 
Ursa Major, or the Great Bear, 330. 

nebula in, 386. 
Ursa Minor, or the Little Bear, 330. 



V. 

Vega, star in Lyra, 336, 365, 370. 
Venus, apparent size of, 231. 

atmosphere of, 234. 

brilliancy of, 232. 

distance of, 231. 

elongation of, 231. 

orbit of, 230. 

periods of, 232. 

volume of, 231. 

transits ol, 145, 234. 
Vernier, the, 15. 
Virgo, or the Virgin, 338. 
Vesta, the planet, 241. 
Vulcan, the planet, 230. 



Y. 

Year, the, 78. 

anomalistic, 79. 
Julian, 80. 
sidereal, 79. 
tropical, 79. 



Zenith, the, 6. 

distance, 12. 
Zodiac, the, 32. 
Zodiacal constellations 

light, 318. 
Zones, 61. 



T 613 



I Ml 



I 





